Efficacy and safety of trimodulin in patients with severe COVID-19: results from a randomised, placebo-controlled, double-blind, multicentre, phase II trial (ESsCOVID)

BACKGROUND

Trimodulin (human polyvalent immunoglobulin [Ig] M ~ 23%, IgA ~ 21%, IgG ~ 56% preparation) has previously been associated with a lower mortality rate in a subpopulation of patients with severe community-acquired pneumonia on invasive mechanical ventilation (IMV) and with clear signs of inflammation. The hypothesis for the ESsCOVID trial was that trimodulin may prevent inflammation-driven progression of severe coronavirus disease 2019 (COVID-19) to critical disease or even death.

METHODS

Adults with severe COVID-19 were randomised to receive intravenous infusions of trimodulin or placebo for 5 consecutive days in addition to standard of care. The primary efficacy endpoint was a composite of clinical deterioration (Days 6-29) and 28-day all-cause mortality (Days 1-29).

RESULTS

One-hundred-and-sixty-six patients received trimodulin (n = 84) or placebo (n = 82). Thirty-three patients died, nine during the treatment phase. Overall, 84.9% and 76.5% of patients completed treatment and follow-up, respectively. The primary efficacy endpoint was reported in 33.3% of patients on trimodulin and 34.1% of patients on placebo (P = 0.912). No differences were observed in the proportion of patients recovered on Day 29, days of invasive mechanical ventilation, or intensive care unit-free days. Rates of treatment-emergent adverse events were comparable. A post hoc analysis was conducted in patients with early systemic inflammation by excluding those with high CRP (> 150 mg/L) and/or D-dimer (≥ 3 mg/L) and/or low platelet counts (< 130 × 109/L) at baseline. Forty-seven patients in the trimodulin group and 49 in the placebo group met these criteria. A difference of 15.5 percentage points in clinical deterioration and mortality was observed in favour of trimodulin (95% confidence interval: -4.46, 34.78; P = 0.096).

CONCLUSION

Although there was no difference in the primary outcome in the overall population, observations in a subgroup of patients with early systemic inflammation suggest that trimodulin may have potential in this setting that warrants further investigation. ESSCOVID WAS REGISTERED PROSPECTIVELY AT CLINICALTRIALS.GOV ON OCTOBER 6, 2020.: NCT04576728.

How Membrane Phospholipids Containing Long-Chain Polyunsaturated Fatty Acids and Their Oxidation Products Orchestrate Lipid Raft Dynamics to Control Inflammation

BACKGROUND

Long-chain PUFA (LC-PUFA) influence varying aspects of inflammation. One mechanism by which they regulate inflammation is by controlling the size and molecular composition of lipid rafts. Lipid rafts are sphingolipid/cholesterol-enriched plasma membrane microdomains that compartmentalize signaling proteins and thereby control downstream inflammatory gene expression and cytokine production.

OBJECTIVES

This review summarizes developments in our understanding of how LC-PUFA acyl chains of phospholipids, in addition to oxidized derivatives of LC-PUFAs such as oxidized 1-palmitoyl-2-arachidonyl-phosphatidylcholine (oxPAPC), manipulate formation of lipid rafts and thereby inflammation.

METHODS

We reviewed the literature, largely from the past 2 decades, on the impact of LC-PUFA acyl chains and oxidized products of LC-PUFAs on lipid raft biophysical organization of myeloid and lymphoid cells. The majority of the studies are based on rodent or cellular experiments with supporting mechanistic studies using biomimetic membranes and molecular dynamic simulations. These studies have focused largely on the LC-PUFA docosahexaenoic acid, with some studies addressing eicosapentaenoic acid. A few studies have investigated the role of oxidized phospholipids on rafts.

RESULTS

The biophysical literature suggests a model in which n-3 LC-PUFAs, in addition to oxPAPC, localize predominately to nonraft regions and impart a disordering effect in this environment. Rafts become larger because of the ensuing increase in the difference in order between raft and nonrafts. Biochemical studies suggest that some n-3 LC-PUFAs can be found within rafts. This deviation from homeostasis is a potential trigger for controlling aspects of innate and adaptive immunity.

CONCLUSION

Overall, select LC-PUFA acyl chains and oxidized acyl chains of phospholipids control lipid raft dynamics and downstream inflammation. Gaps in knowledge remain, particularly on underlying molecular mechanisms by which plasma membrane receptor organization is controlled in response to oxidized LC-PUFA acyl chains of membrane phospholipids. Validation in humans is also an area for future study.

Secreted IgM modulates IL-10 expression in B cells

IL-10+ B cells are critical for immune homeostasis and restraining immune responses in infection, cancer, and inflammation; however, the signals that govern IL-10+ B cell differentiation are ill-defined. Here we find that IL-10+ B cells expand in mice lacking secreted IgM ((s)IgM-/-) up to 10-fold relative to wildtype (WT) among all major B cell and regulatory B cell subsets. The IL-10+ B cell increase is polyclonal and presents within 24 hours of birth. In WT mice, sIgM is produced prenatally and limits the expansion of IL-10+ B cells. Lack of the high affinity receptor for sIgM, FcμR, in B cells translates into an intermediate IL-10+ B cell phenotype relative to WT or sIgM-/- mice. Our study thus shows that sIgM regulates IL-10 programming in B cells in part via B cell-expressed FcμR, thereby revealing a function of sIgM in regulating immune homeostasis.

Natural autoimmunity in oligoarticular juvenile idiopathic arthritis

BACKGROUND

Oligoarticular juvenile idiopathic arthritis (oligo-JIA) is considered as an antigen-driven lymphocyte-mediated autoimmune disease. Natural antibodies (NAbs) are pre-immune antibodies produced in the absence of exogenous antigen stimulation, participating in both, innate and adaptive immunity. Considering their major immunoregulatory role in homeostasis and autoimmune pathogenesis, we designed this study to further elucidate their role in oligo-JIA pathogenesis.

METHODS

Seventy children with persistent oligo-JIA and 20 healthy matched controls were enrolled in the study. Serum IgM and IgA antibodies against human G-actin, human IgG F(ab΄)2 fragments and the hapten TriNitroPhenol (TNP) as well as the total concentration of serum IgM and IgA were measured by in-house enzyme-immunoassays. Kolmogorov-Smirnov normality test, Kruskal-Wallis H and Mann-Whitney tests were used to assess data distribution, and significant differences of non-parametric data between groups of the study. Backward regression analysis was used to analyze the effect of multiple factors (age, gender, disease activity, anti-nuclear antibody positivity, presence of uveitis) on continuous dependent variables (activities and activity/ concentration ratios of IgM and IgA NAbs).

RESULTS

The ratios of IgA anti-TNP, anti-actin and anti-F(ab΄)₂ levels to total serum IgA concentration were found to be significantly increased in patients with oligo-JIA compared to healthy subjects. Significantly elevated levels of IgM anti-TNP antibodies were also found in children with inactive oligo-JIA compared to those of children with active disease and of healthy controls. In the presence of anterior uveitis, IgM anti-TNP levels were significantly higher than in patients without uveitis or in healthy controls. Backward regression analysis revealed that the disease activity and the presence of anterior uveitis independently affect IgM anti-TNP levels.

CONCLUSUIONS

Our findings are in accordance with the hypothesis that NAbs contribute to the pathogenesis of autoimmune diseases and provide additional evidence that disturbances in natural autoimmunity may contribute to the as yet unclarified pathogenesis of oligo-JIA.

B-1 plasma cells require non-cognate CD4 T cell help to generate a unique repertoire of natural IgM

Evolutionarily conserved, "natural" (n)IgM is broadly reactive to both self and foreign antigens. Its selective deficiency leads to increases in autoimmune diseases and infections. In mice, nIgM is secreted independent of microbial exposure to bone marrow (BM) and spleen B-1 cell-derived plasma cells (B-1PC), generating the majority of nIgM, or by B-1 cells that remain non-terminally differentiated (B-1sec). Thus, it has been assumed that the nIgM repertoire is broadly reflective of the repertoire of body cavity B-1 cells. Studies here reveal, however, that B-1PC generate a distinct, oligoclonal nIgM repertoire, characterized by short CDR3 variable immunoglobulin heavy chain regions, 7-8 amino acids in length, some public, many arising from convergent rearrangements, while specificities previously associated with nIgM were generated by a population of IgM-secreting B-1 (B-1sec). BM, but not spleen B-1PC, or B-1sec also required the presence of TCRαβ CD4 T cells for their development from fetal precursors. Together, the studies identify important previously unknown characteristics of the nIgM pool.

A new platform for autoimmune diseases. Inducing tolerance with liposomes encapsulating autoantigens

Autoimmune diseases (AIDs) are caused by the loss of self-tolerance and destruction of tissues by the host's immune system. Several antigen-specific immunotherapies, focused on arresting the autoimmune attack, have been tested in clinical trials with discouraging results. Therefore, there is a need for innovative strategies to restore self-tolerance safely and definitively in AIDs. We previously demonstrated the therapeutic efficacy of phosphatidylserine (PS)-liposomes encapsulating autoantigens in experimental type 1 diabetes and multiple sclerosis. Here, we show that PS-liposomes can be adapted to other autoimmune diseases by simply replacing the encapsulated autoantigen. After administration, they are distributed to target organs, captured by phagocytes and interact with several immune cells, thus exerting a tolerogenic and immunoregulatory effect. Specific PS-liposomes demonstrate great preventive and therapeutic efficacy in rheumatoid arthritis and myasthenia gravis. Thus, this work highlights the therapeutic potential of a platform for several autoimmunity settings, which is specific, safe, and with long-term effects.

The Lack of Natural IgM Increases Susceptibility and Impairs Anti-Vi Polysaccharide IgG Responses in a Mouse Model of Typhoid

Circulating IgM present in the body prior to any apparent Ag exposure is referred to as natural IgM. Natural IgM provides protective immunity against a variety of pathogens. Salmonella enterica serovar Typhi (S. Typhi) is the causative agent of typhoid fever in humans. Because mice are not permissive to S. Typhi infection, we employed a murine model of typhoid using S. enterica serovar Typhimurium expressing the Vi polysaccharide (ViPS) of S. Typhi (S. Typhimurium strain RC60) to evaluate the role of natural IgM in pathogenesis. We found that natural mouse IgM binds to S. Typhi and S. Typhimurium. The severity of S. Typhimurium infection in mice is dependent on presence of the natural resistance-associated macrophage protein 1 (Nramp1) allele; therefore, we infected mice deficient in secreted form of IgM (sIgM) on either a Nramp1-resistant (129S) or -susceptible (C57BL/6J) background. We found that the lack of natural IgM results in a significantly increased susceptibility and an exaggerated liver pathology regardless of the route of infection or the Nramp1 allele. Reconstitution of sIgM-/- mice with normal mouse serum or purified polyclonal IgM restored the resistance to that of sIgM+/+ mice. Furthermore, immunization of sIgM-/- mice with heat-killed S. Typhi induced a significantly reduced anti-ViPS IgG and complement-dependent bactericidal activity against S. Typhi in vitro, compared with that of sIgM+/+ mice. These findings indicate that natural IgM is an important factor in reducing the typhoid severity and inducing an optimal anti-ViPS IgG response to vaccination.

The OSE complotype and its clinical potential

Cellular death, aging, and tissue damage trigger inflammation that leads to enzymatic and non-enzymatic lipid peroxidation of polyunsaturated fatty acids present on cellular membranes and lipoproteins. This results in the generation of highly reactive degradation products, such as malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), that covalently modify free amino groups of proteins and lipids in their vicinity. These newly generated neoepitopes represent a unique set of damage-associated molecular patterns (DAMPs) associated with oxidative stress termed oxidation-specific epitopes (OSEs). OSEs are enriched on oxidized lipoproteins, microvesicles, and dying cells, and can trigger sterile inflammation. Therefore, prompt recognition and removal of OSEs is required to maintain the homeostatic balance. This is partially achieved by various humoral components of the innate immune system, such as natural IgM antibodies, pentraxins and complement components that not only bind OSEs but in some cases modulate their pro-inflammatory potential. Natural IgM antibodies are potent complement activators, and 30% of them recognize OSEs such as oxidized phosphocholine (OxPC-), 4-HNE-, and MDA-epitopes. Furthermore, OxPC-epitopes can bind the complement-activating pentraxin C-reactive protein, while MDA-epitopes are bound by C1q, C3a, complement factor H (CFH), and complement factor H-related proteins 1, 3, 5 (FHR-1, FHR-3, FHR-5). In addition, CFH and FHR-3 are recruited to 2-(ω-carboxyethyl)pyrrole (CEP), and full-length CFH also possesses the ability to attenuate 4-HNE-induced oxidative stress. Consequently, alterations in the innate humoral defense against OSEs predispose to the development of diseases associated with oxidative stress, as shown for the prototypical OSE, MDA-epitopes. In this mini-review, we focus on the mechanisms of the accumulation of OSEs, the pathophysiological consequences, and the interactions between different OSEs and complement components. Additionally, we will discuss the clinical potential of genetic variants in OSE-recognizing complement proteins – the OSE complotype - in the risk estimation of diseases associated with oxidative stress.

Secondary Lymphoid Organs in Mesenchymal Stromal Cell Therapy: More Than Just a Filter

Mesenchymal stromal cells (MSCs) have demonstrated therapeutic potential in inflammatory models of human disease. However, clinical translation has fallen short of expectations, with many trials failing to meet primary endpoints. Failure to fully understand their mechanisms of action is a key factor contributing to the lack of successful commercialisation. Indeed, it remains unclear how the long-ranging immunomodulatory effects of MSCs can be attributed to their secretome, when MSCs undergo apoptosis in the lung shortly after intravenous infusion. Their apoptotic fate suggests that efficacy is not based solely on their viable properties, but also on the immune response to dying MSCs. The secondary lymphoid organs (SLOs) orchestrate immune responses and play a key role in immune regulation. In this review, we will discuss how apoptotic cells can modify immune responses and highlight the importance of MSC-immune cell interactions in SLOs for therapeutic outcomes.

The cross-talk between soluble "Find me" and "Keep out" signals as an initial step in regulating efferocytosis

The rapid clearance of apoptotic cells (ACs), known as efferocytosis, prompts the inhibition of inflammatory responses and autoimmunity and maintains homeostatic cell turnover by controlling the release of intracellular contents. The fast clearance of ACs requires professional and nonprofessional phagocytic cells that can accurately and promptly recognize ACs and migrate towards them. Cells undergoing apoptosis alarm their presence by releasing special soluble chemotactic factors, such as lactoferrin, that act as "Find me," "Keep out," or "Stay away" signals to recruit phagocytic cells, such as macrophages or prevent granulocyte migration. Efferocytosis effectively serves to prevent damage-associated molecular pattern release and secondary necrosis and inhibit inflammation/autoimmunity at the very first step. Since less attention has been given to the cross-talk and balance of "Find me" and "Keep out" signals released from ACs in efferocytosis, we set out to investigate the current knowledge of the roles of "Find me" and "Keep out" signals in the efferocytosis process.

Approaches for Controlling Antibody-Mediated Allograft Rejection Through Targeting B Cells

Both acute and chronic antibody-mediated allograft rejection (AMR), which are directly mediated by B cells, remain difficult to treat. Long-lived plasma cells (LLPCs) in bone marrow (BM) play a crucial role in the production of the antibodies that induce AMR. However, LLPCs survive through a T cell-independent mechanism and resist conventional immunosuppressive therapy. Desensitization therapy is therefore performed, although it is accompanied by severe side effects and the pathological condition may be at an irreversible stage when these antibodies, which induce AMR development, are detected in the serum. In other words, AMR control requires the development of a diagnostic method that predicts its onset before LLPC differentiation and enables therapeutic intervention and the establishment of humoral immune monitoring methods providing more detailed information, including individual differences in the susceptibility to immunosuppressive agents and the pathological conditions. In this study, we reviewed recent studies related to the direct or indirect involvement of immunocompetent cells in the differentiation of naïve-B cells into LLPCs, the limitations of conventional methods, and the possible development of novel control methods in the context of AMR. This information will significantly contribute to the development of clinical applications for AMR and improve the prognosis of patients who undergo organ transplantation.

Phagocytic clearance of apoptotic, necrotic, necroptotic and pyroptotic cells

Although millions of cells in the human body will undergo programmed cell death each day, dying cells are rarely detected under homeostatic settings in vivo. The swift removal of dying cells is due to the rapid recruitment of phagocytes to the site of cell death which then recognise and engulf the dying cell. Apoptotic cell clearance - the engulfment of apoptotic cells by phagocytes - is a well-defined process governed by a series of molecular factors including 'find-me', 'eat-me', 'don't eat-me' and 'good-bye' signals. However, in recent years with the rapid expansion of the cell death field, the removal of other necrotic-like cell types has drawn much attention. Depending on the type of death, dying cells employ different mechanisms to facilitate engulfment and elicit varying functional impacts on the phagocyte, from wound healing responses to inflammatory cytokine secretion. Nevertheless, despite the mechanism of death, the clearance of dying cells is a fundamental process required to prevent the uncontrolled release of pro-inflammatory mediators and inflammatory disease. This mini-review summarises the current understandings of: (i) apoptotic, necrotic, necroptotic and pyroptotic cell clearance; (ii) the functional consequences of dying cell engulfment and; (iii) the outstanding questions in the field.

Mini-Review: The Administration of Apoptotic Cells for Treating Rheumatoid Arthritis: Current Knowledge and Clinical Perspectives

Rheumatoid arthritis (RA) is a chronic immune-mediated disease managed by conventional synthetic drugs, such as methotrexate (MTX), and targeted drugs including biological agents. Cell-based therapeutic approaches are currently developed in RA, mainly mesenchymal stroma cell-based approaches. Early-stage apoptotic cells possess direct and indirect anti-inflammatory properties. During the elimination of dying cells (a process called efferocytosis), specific mechanisms operate to control immune responses. There are compelling evidences in experimental models of arthritis indicating that apoptotic cell administration may benefit joint inflammation, and may even have therapeutic effects on arthritis. Additionally, it has been demonstrated that apoptotic cells could be administered with standard treatments of RA, such as MTX or TNF inhibitors (TNFi), given even a synergistic response with TNFi. Interestingly, apoptotic cell infusion has been successfully experienced to prevent acute graft-vs.-host disease after hematopoietic cell transplantation in patients with hematologic malignancies, with a good safety profile. In this mini-review, the apoptotic cell-based therapy development in arthritis is discussed, as well as its transfer in the short-term to an innovative treatment for patients with RA. The use of apoptotic cell-derived factors, including secretome or phosphatidylserine-containing liposomes, in RA are also discussed.

Antinuclear Autoantibodies in Health: Autoimmunity Is Not a Synonym of Autoimmune Disease

The incidence of autoimmune diseases is increasing. Antinuclear antibody (ANA) testing is a critical tool for their diagnosis. However, ANA prevalence in healthy persons has increased over the last decades, especially among young people. ANA in health occurs in low concentrations, with a prevalence up to 50% in some populations, which demands a cutoff revision. This review deals with the origin and probable physiological or compensatory function of ANA in health, according to the concept of immunological clearance, theory of autoimmune regulation of cell functions, and the concept of functional autoantibodies. Considering ANA titers ≤1:320 as a serological marker of autoimmune diseases seems inappropriate. The role of anti-DFS70/LEDGFp75 autoantibodies is highlighted as a possible anti-risk biomarker for autoimmune rheumatic disorders. ANA prevalence in health is different in various regions due to several underlying causes discussed in the review, all influencing additive combinations according to the concept of the mosaic of autoimmunity. Not only are titers, but also HEp-2 IFA) staining patterns, such as AC-2, important. Accepting autoantibodies as a kind of bioregulator, not only the upper, but also the lower borders of their normal range should be determined; not only their excess, but also a lack of them or "autoimmunodeficiency" could be the reason for disorders.

Apoptotic Donor Cells in Transplantation

Despite significant advances in prevention and treatment of transplant rejection with immunosuppressive medications, we continue to face challenges of long-term graft survival, detrimental medication side effects to both the recipient and transplanted organ together with risks for opportunistic infections. Transplantation tolerance has so far only been achieved through hematopoietic chimerism, which carries with it a serious and life-threatening risk of graft versus host disease, along with variability in persistence of chimerism and uncertainty of sustained tolerance. More recently, numerous in vitro and in vivo studies have explored the therapeutic potential of silent clearance of apoptotic cells which have been well known to aid in maintaining peripheral tolerance to self. Apoptotic cells from a donor not only have the ability of down regulating the immune response, but also are a way of providing donor antigens to recipient antigen-presenting-cells that can then promote donor-specific peripheral tolerance. Herein, we review both laboratory and clinical evidence that support the utility of apoptotic cell-based therapies in prevention and treatment of graft versus host disease and transplant rejection along with induction of donor-specific tolerance in solid organ transplantation. We have highlighted the potential limitations and challenges of this apoptotic donor cell-based therapy together with ongoing advancements and attempts made to overcome them.

Relationship between natural and infection-induced antibodies in systemic autoimmune diseases (SAD): SLE, SSc and RA

Infection or vaccine-induced T cell-dependent immune response and the subsequent high-affinity neutralizing antibody production have been extensively studied, while the connection between natural autoantibodies (nAAbs) and disease-specific antibodies has not been thoroughly investigated. Our goal was to find the relationship between immunoglobulin (Ig)M and IgG isotype nAAbs and infection or vaccine-induced and disease-related autoantibody levels in systemic autoimmune diseases (SAD). A previously described indirect enzyme-linked immunosorbent assay (ELISA) test was used for detection of IgM/IgG nAAbs against citrate synthase (anti-CS) and F4 fragment (anti-F4) of DNA topoisomerase I in 374 SAD samples, with a special focus on systemic lupus erythematosus (SLE) (n = 92), rheumatoid arthritis (n = 73) and systemic sclerosis (n = 157) disease groups. Anti-measles IgG and anti-dsDNA IgG/IgM autoantibodies were measured using commercial and in-house indirect ELISA tests. In all SAD groups the anti-measles IgG-seropositive cases showed significantly higher anti-CS IgG titers (P = 0·011). In anti-dsDNA IgG-positive SLE patients, we detected significantly higher levels of anti-CS and anti-F4 IgG nAAbs (P = 0·001 and < 0·001, respectively). Additionally, we found increased levels of IgM isotypes of anti-CS and anti-F4 nAAbs in anti-dsDNA IgM-positive SLE patients (P = 0·002 and 0·016, respectively). The association between IgG isotypes of pathogen- or autoimmune disease-related antibodies and the IgG nAAbs may underscore the immune response-inducible nature of the diseases investigated. The relationship between protective anti-dsDNA IgM and the IgM isotype of anti-F4 and anti-CS may provide immunoserological evidence for the beneficial roles of nAAbs in SLE patients.

Characteristics of Immunoglobulin M Type Antibodies of Different Origins from the Immunologic and Clinical Viewpoints and Their Application in Controlling Antibody-Mediated Allograft Rejection

Antibody-mediated allograft rejection (AMR) hinders patient prognosis after organ transplantation. Current studies concerning AMR have mainly focused on the diagnostic value of immunoglobulin G (IgG)-type donor-specific antihuman leukocyte antigen antibodies (DSAs), primarily because of their antigen specificity, whereas the clinical significance of immunoglobulin M (IgM)-type DSAs has not been thoroughly investigated in the context of organ transplantation because of their nonspecificity against antigens. Although consensus regarding the clinical significance and role of IgM antibodies is not clear, as discussed in this review, recent findings strongly suggest that they also have a huge potential in novel diagnostic as well as therapeutic application for the prevention of AMR. Most serum IgM antibodies are known to comprise natural antibodies with low affinity toward antigens, and this is derived from B-1 cells (innate B cells). However, some of the serum IgM-type antibodies reportedly also produced by B-2 cells (conventional B cells). The latter are known to have a high affinity for donor-specific antigens. In this review, we initially discuss how IgM-type antibodies of different origins participate in the pathology of various diseases, directly or through cell surface receptors, complement activation, or cytokine production. Then, we discuss the clinical applicability of B-1 and B-2 cell-derived IgM-type antibodies for controlling AMR with reference to the involvement of IgM antibodies in various pathological conditions.

Autoantigen-specific immune tolerance in pathological and physiological cell death: Nanotechnology comes into view

Apoptotic cells are tolerogenic and can present self-antigens in the absence of inflammation, to antigen-presenting cells by the process of efferocytosis, resulting in anergy and depletion of immune effector cells. This tolerance is essential to maintain immune homeostasis and prevent systemic autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythematosus. Consequently, effective efferocytosis can result in the induction of immune tolerance mediated via triggering modulatory lymphocytes and anti-inflammatory responses. Furthermore, several distinct soluble factors, receptors and pathways have been found to be involved in the efferocytosis, which are able to regulate immune tolerance by lessening antigen presentation, inhibition of T-cell proliferation and induction of regulatory T-cells. Some newly developed nanotechnology-based approaches can induce antigen-specific immunological tolerance without any systemic immunosuppression. These strategies have been explored to reverse autoimmune responses induced against various protein antigens in different diseases. In this review, we describe some nanotechnology-based approaches for the maintenance of self-tolerance using the apoptotic cell clearance process (efferocytosis) that may be able to induce immune tolerance and treat autoimmune diseases.

IL-4 Treatment Mitigates Experimental Cerebral Malaria by Reducing Parasitemia, Dampening Inflammation, and Lessening the Cytotoxicity of T Cells

Cytokine responses to malaria play important roles in both protective immunity development and pathogenesis. Although the roles of cytokines such as TNF-α, IL-12, IFN-γ, and IL-10 in immunity and pathogenesis to the blood stage malaria are largely known, the role of IL-4 remains less understood. IL-4 targets many cell types and induces multiple effects, including cell proliferation, gene expression, protection from apoptosis, and immune regulation. Accordingly, IL-4 has been exploited as a therapeutic for several inflammatory diseases. Malaria caused by Plasmodium falciparum manifests in many organ-specific fatal pathologies, including cerebral malaria (CM), driven by a high parasite load, leading to parasite sequestration in organs and consequent excessive inflammatory responses and endothelial damage. We investigated the therapeutic potential of IL-4 against fatal malaria in Plasmodium berghei ANKA-infected C57BL/6J mice, an experimental CM model. IL-4 treatment significantly reduced parasitemia, CM pathology, and mortality. The therapeutic effect of IL-4 is mediated through multiple mechanisms, including enhanced parasite clearance mediated by upregulation of phagocytic receptors and increased IgM production, and decreased brain inflammatory responses, including reduced chemokine (CXCL10) production, reduced chemokine receptor (CXCR3) and adhesion molecule (LFA-1) expression by T cells, and downregulation of cytotoxic T cell lytic potential. IL-4 treatment markedly reduced the infiltration of CD8⁺ T cells and brain pathology. STAT6, PI3K-Akt-NF-κB, and Src signaling mediated the cellular and molecular events that contributed to the IL-4-dependent decrease in parasitemia. Overall, our results provide mechanistic insights into how IL-4 treatment mitigates experimental CM and have implications in developing treatment strategies for organ-specific fatal malaria.

Natural Antibodies: from First-Line Defense Against Pathogens to Perpetual Immune Homeostasis

Natural antibodies (nAbs) are most commonly defined as immunoglobulins present in the absence of pathological conditions or deliberate immunizations. Occurrence of nAbs in germ- and antigen-free mice suggest that their production is driven, at least in part, by self-antigens. Accordingly, nAbs are constituted of natural autoantibodies (nAAbs), and can belong to the IgM, IgG, or IgA subclasses. These nAbs provide immediate protection against infection while the adaptive arm of the immune system mounts a specific and long-term response. Beyond immediate protection from infection, nAbs have been shown to play various functional roles in the immune system, which include clearance of apoptotic debris, suppression of autoimmune and inflammatory responses, regulation of B cell responses, selection of the B cell repertoires, and regulation of B cell development. These various functions of nAbs are afforded by their reactivity, which is broad, cross-reactive, and shown to recognize evolutionarily fixed epitopes shared between foreign and self-antigens. Furthermore, nAbs have unique characteristics that also contribute to their functional roles and set them apart from antigen-specific antibodies. In further support for the role of nAbs in the protection against infections and in the maintenance of immune homeostasis, the therapeutic preparation of polyclonal immunoglobulins, intravenous immunoglobulin (IVIG), rich in nAbs is commonly used in the replacement therapy of primary and secondary immunodeficiencies and in the immunotherapy of a large number of autoimmune and inflammatory diseases. Here, we review several topics on nAbs features and functions, and therapeutic applications in human diseases.

Anti-Pituitary and Anti-Hypothalamus Autoantibody Associations with Inflammation and Persistent Hypogonadotropic Hypogonadism in Men with Traumatic Brain Injury

Traumatic brain injury (TBI) and can lead to persistent hypogonadotropic hypogonadism (PHH) and poor outcomes. We hypothesized that autoimmune and inflammatory mechanisms contribute to PHH pathogenesis. Men with moderate-to-severe TBI (n = 143) were compared with healthy men (n = 39). The TBI group provided blood samples 1-12 months post-injury (n = 1225). TBI and healthy control (n = 39) samples were assayed for testosterone (T) and luteinizing hormone (LH) to adjudicate PHH status. TBI samples 1-6 months post-injury and control samples were assayed for immunoglobulin M (IgM)/immunoglobulin G (IgG) anti-pituitary autoantibodies (APA) and anti-hypothalamus autoantibodies (AHA). Tissue antigen specificity for APA and AHA was confirmed via immunohistochemistry (IHC). IgM and IgG autoantibodies for glial fibrillary acid protein (GFAP) (AGA) were evaluated to gauge APA and AHA production as a generalized autoimmune response to TBI and to evaluate the specificity of APA and AHA to PHH status. An inflammatory marker panel was used to assess relationships to autoantibody profiles and PHH status. Fifty-one men with TBI (36%) had PHH. An age-related decline in T levels by both TBI and PHH status were observed. Injured men had higher APA IgM, APA IgG, AHA IgM, AHA IgG, AGA IgM, and AGA IgG than controls (p < 0.0001 all comparisons). However, only APA IgM (p = 0.03) and AHA IgM (p = 0.03) levels were lower in the PHH than in the non-PHH group in multivariate analysis. There were no differences in IgG levels by PHH status. Multiple inflammatory markers were positively correlated with IgM autoantibody production. PHH was associated with higher soluble tumor-necrosis-factor receptors I/II, (sTNFRI, sTNFRII), regulated on activation, normal T-cell expressed and secreted (RANTES) and soluble interleukin-2-receptor-alpha (sIL-2Rα) levels. Higher IgM APA, and AHA, but not AGA, in the absence of PHH may suggest a beneficial or reparative role for neuroendocrine tissue-specific IgM autoantibody production against PHH development post-TBI.

Secreted IgM: New tricks for an old molecule

Secreted IgM (sIgM) is a multifunctional evolutionary conserved antibody that is critical for the maintenance of tissue homeostasis as well as the development of fully protective humoral responses to pathogens. Constitutive secretion of self- and polyreactive natural IgM, produced mainly by B-1 cells, provides a circulating antibody that engages with autoantigens as well as invading pathogens, removing apoptotic and other cell debris and initiating strong immune responses. Pathogen-induced IgM production by B-1 and conventional B-2 cells strengthens this early, passive layer of IgM-mediated immune defense and regulates subsequent IgG production. The varied effects of secreted IgM on immune homeostasis and immune defense are facilitated through its binding to numerous different cell types via different receptors. Recent studies identified a novel function for pentameric IgM, namely as a transporter for the effector protein ″apoptosis-inhibitor of macrophages″ (AIM/CD5L). This review aims to provide a summary of the known functions and effects of sIgM on immune homeostasis and immune defense, and its interaction with its various receptors, and to highlight the many critical immune regulatory functions of this ancient and fascinating immunoglobulin.

B cell receptor and Toll-like receptor signaling coordinate to control distinct B-1 responses to both self and the microbiota

B-1a cells play an important role in mediating tissue homeostasis and protecting against infections. They are the main producers of 'natural' IgM, spontaneously secreted serum antibodies predominately reactive to self antigens, like phosphatidylcholine (PtC), or antigens expressed by the intestinal microbiota. The mechanisms that regulate the B-1a immunoglobulin (Ig) repertoire and their antibody secretion remain poorly understood. Here, we use a novel reporter mouse to demonstrate that production of self- and microbiota-reactive antibodies is linked to BCR signaling in B-1a cells. Moreover, we show that Toll-like receptors (TLRs) are critical for shaping the Ig repertoire of B-1a cells as well as regulating their antibody production. Strikingly, we find that both the colonization of a microbiota as well as microbial-sensing TLRs are required for anti-microbiota B-1a responses, whereas nucleic-acid sensing TLRs are required for anti-PtC responses, demonstrating that linked activation of BCR and TLRs controls steady state B-1a responses to both self and microbiota-derived antigens.

Treatment of Borrelia burgdorferi-Infected Mice with Apoptotic Cells Attenuates Lyme Arthritis via PPAR-γ

Infection of mice with Borrelia burgdorferi causes an inflammatory arthritis that peaks 3-4 wk postinfection and then spontaneously resolves. Although the recruitment of neutrophils is known to drive the development of arthritis, mechanisms of disease resolution remain unclear. Efficient clearance of apoptotic cells (AC) is likely an important component of arthritis resolution. In this article, we show the number of AC increases in the joints of B. burgdorferi-infected mice around day 21 postinfection and peaks around day 28. Injection of AC directly into the ankles of B. burgdorferi-infected mice limited ankle swelling but had no effect on spirochete clearance or arthritis severity scores. In vitro, addition of AC to bone marrow macrophage cultures decreased B. burgdorferi-induced TNF-α and KC and increased IL-10. In addition, phagocytosis of B. burgdorferi and neutrophil migration to LTB₄ were inhibited by AC. Exogenous AC caused an increase in peroxisome proliferator-activated receptor-γ (PPAR-γ) expression both in vitro and in vivo during B. burgdorferi infection. The PPAR-γ agonist rosiglitazone elicited similar changes in macrophage cytokine production and neutrophil migration as exogenous AC. Addition of the PPAR-γ antagonist GW 9662 abrogated the effects of AC in vitro. Injection of rosiglitazone directly into the tibiotarsal joints of B. burgdorferi-infected mice decreased ankle swelling and immune cell recruitment, similar to the injection of AC. These results suggest that clearance of AC plays a role in the resolution of inflammation during experimental Lyme arthritis through the activation of PPAR-γ. PPAR-γ agonists, such as rosiglitazone, may therefore be effective treatments for inducing arthritis resolution.

Natural antibodies - facts known and unknown

In contrast to adaptive antibodies, natural antibodies are present in a non-immunised organism from birth, and they do not include anti-Gal antibodies and/or anti-Gal natural antibodies, which are developed as a result of the effect of the α-Gal epitope and physiological flora. Natural antibodies are the first line of the organism’s defence before the formation of the immunity created via the stimulation of elements that determine specific and non-specific immunity. This is especially important in the case of infants. Despite the fact that natural antibodies differ in their function from adaptive antibodies, they are polyreactive and they detect autoantigens and new antigenic determinants. Natural antibodies are formed from the subpopulation of B lymphocytes, mainly B1 lymphocytes and B lymphocytes of the marginal zone. This phenomenon is supported by the fact that when the quantity of these cells in the organism decreases, which happens with age, the level of natural antibodies also decreases and the risk of illnesses of old age becomes higher. During ontogenesis, these antibodies participate in many physiological processes, including the “support” of the immune system and homeostasis, the prevention of inflammation, infections and other pathological states, such as autoimmune and cardiovascular diseases, or the process of carcinogenesis. The best known natural antibody is IgM, but the role of IgGs and IgAs is also considered important. Nowadays, many researchers also mention intravenous immunoglobulins, which are used in the treatment of numerous illnesses, and there are discussions on the possibility of increasing their potential if they were based on natural antibodies.

Healthy Donor Polyclonal IgMs Diminish B-Lymphocyte Autoreactivity, Enhance Regulatory T-Cell Generation, and Reverse Type 1 Diabetes in NOD Mice

Autoimmune diseases such as type 1 diabetes (T1D) arise from unrestrained activation of effector lymphocytes that destroy target tissues. Many efforts have been made to eliminate these effector lymphocytes, but none has produced a long-term cure. An alternative to depletion therapy is to enhance endogenous immune regulation. Among these endogenous alternatives, naturally occurring Igs have been applied for inflammatory disorders but have lacked potency in antigen-specific autoimmunity. We hypothesized that naturally occurring polyclonal IgMs, which represent the majority of circulating, noninduced antibodies but are present only in low levels in therapeutic Ig preparations, possess the most potent capacity to restore immune homeostasis. Treatment of diabetes-prone NOD mice with purified IgM isolated from Swiss Webster (SW) mice (nIgM_SW) reversed new-onset diabetes, eliminated autoreactive B lymphocytes, and enhanced regulatory T-cell (Treg) numbers both centrally and peripherally. Conversely, IgM from prediabetic NOD mice could not restore this endogenous regulation, which represents an unrecognized component of T1D pathogenesis. Of note, IgM derived from healthy human donors was similarly able to expand human CD4 Tregs in humanized mice and produced permanent diabetes protection in treated NOD mice. Overall, these studies demonstrate that a potent, endogenous regulatory mechanism, nIgM, is a promising option for reversing autoimmune T1D in humans.

Anti-TIM-1 Monoclonal Antibody (RMT1-10) Attenuates Atherosclerosis By Expanding IgM-producing B1a Cells

Background

Peritoneal B1a cells attenuate atherosclerosis by secreting natural polyclonal immunoglobulin M (IgM). Regulatory B cells expressing T‐cell immunoglobulin mucin domain‐1 (TIM‐1) expanded through TIM‐1 ligation by anti‐TIM‐1 monoclonal antibody (RMT1‐10) induces immune tolerance.

Methods and Results

We examined the capacity of RMT1‐10 to expand peritoneal B1a cells to prevent atherosclerosis development and retard progression of established atherosclerosis. RMT1‐10 treatment selectively doubled peritoneal B1a cells, tripled TIM‐1⁺ B1a cells and increased TIM‐1⁺IgM⁺interleukin (IL)‐10⁺ by 3‐fold and TIM‐1⁺IgM⁺IL‐10⁻ B1a cells by 2.5‐fold. Similar expansion of B1a B cells was observed in spleens. These effects reduced atherosclerotic lesion size, increased plasma IgM and lesion IgM deposits, and decreased oxidatively modified low‐density lipoproteins in lesions. Lesion CD4⁺ and CD8⁺ T cells, macrophages and monocyte chemoattractant protein‐1, vascular cell adhesion molecule‐1, expression of proinflammatory cytokines monocyte chemoattractant protein‐1, vascular cell adhesion molecule‐1, IL1β, apoptotic cell numbers and necrotic cores were also reduced. RMT1‐10 treatment failed to expand peritoneal B1a cells and reduce atherosclerosis after splenectomy that reduces B1a cells, indicating that these effects are B1a cell‐dependent. Apolipoprotein E‐KO mice fed a high‐fat diet for 6 weeks before treatment with RMT1‐10 also increased TIM‐1⁺IgM⁺IL‐10⁺ and TIM‐1⁺IgM⁺IL‐10⁻ B1a cells and IgM levels and attenuated progression of established atherosclerosis.

Conclusions

RMT1‐10 treatment attenuates atherosclerosis development and progression by selectively expanding IgM producing atheroprotective B1a cells. Antibody‐based in vivo expansion of B1a cells could be an attractive approach for treating atherosclerosis.

Immune Tolerance to Apoptotic Self Is Mediated Primarily by Regulatory B1a Cells

The chronic autoimmune inflammatory diseases, systemic lupus erythematosus and Sjogren's syndrome, develop when tolerance to apoptotic cells (ACs) is lost. We have previously reported that this tolerance is maintained by innate-like, IL-10 secreting regulatory B cells. Two questions remained. First, do these regulatory B cells belong predominantly to a single subset of steady-state B cells and second, what is their specificity? We report here that innate-like B cells with markers characteristic for B1a cells (CD43+veCD19hiCD5+veIgMhiIgDlo) constitute 80% of splenic and 96% of peritoneal B cells that respond to ACs by secreting IL-10. AC responsive B1a cells secrete self-reactive natural antibodies (NAbs) and IL-10, which is augmented by toll-like receptor (TLR) 7 or TLR9 stimulation. In so doing, they both accelerate the clearance of dying cells by macrophages and inhibit their potential to mount proinflammatory immune responses. While B1a cells make prolonged contact with ACs, they do not require TIM1 or complement to mediate their regulatory function. In an animal model of neural inflammation (experimental autoimmune encephalomyelitis), just 105 activated B1a B cells was sufficient to restrain inflammation. Activated B1a B cells also induced antigen-specific T cells to secrete IL-10. Hence, regulatory B1a cells specifically recognize and augment tolerance to apoptotic self via IL-10 and NAbs; but once activated, can also prevent autoimmune mediated inflammation.

Linking autoimmunity to the origin of the adaptive immune system

In jawed vertebrates, the adaptive immune system (AIS) cooperates with the innate immune system (IIS) to protect hosts from infections. Although targeting non-self-components, the AIS also generates self-reactive antibodies which, when inadequately counter-selected, can give rise to autoimmune diseases (ADs). ADs are on the rise in western countries. Why haven’t ADs been eliminated during the evolution of a ∼500 million-year old system? And why have they become more frequent in recent decades? Self-recognition is an attribute of the phylogenetically more ancient IIS and empirical data compellingly show that some self-reactive antibodies, which are classifiable as elements of the IIS rather then the AIS, may protect from (rather than cause) ADs. Here, we propose that the IIS’s self-recognition system originally fathered the AIS and, as a consequence of this relationship, its activity is dampened in hygienic environments. Rather than a mere breakdown or failure of the mechanisms of self-tolerance, ADs might thus arise from architectural constraints.

Natural IgM and the Development of B Cell-Mediated Autoimmune Diseases

Most serum immunoglobulin M (IgM) is "natural IgM", which is produced apparently spontaneously by a distinct subset of B cells requiring no exogenous antigenic or microbial stimuli. Natural IgM is an evolutionarily conserved molecule and reacts with a variety of epitopes expressed on both self- and non-self antigens. It has long been understood that secreted (s) IgM contributes to the removal of altered self-antigens, such as apoptotic and dying cells. As we outline in this review, it is thought that this sIgM housekeeping function removes potential triggers of autoresponse induction. However, we recently demonstrated an unexpected and distinct role for sIgM in the control of autoreactive B cells: the regulation of bone marrow B cell development. The absence of sIgM blocked pro- to pre- B-cell transition and greatly altered the BCR repertoire of the developing B cells and the peripheral B-cell pools in genetically engineered mice. This finding strongly suggests that IgM is critical for B-cell central tolerance induction. Given that treatment of sIgM-deficient mice with polyclonal IgM corrected these developmental defects, therapeutic application of IgM could be of clinical relevance in the treatment of some B-cell-mediated autoimmune diseases.

Interaction of macrophages with apoptotic cells inhibits transdifferentiation and invasion of lung fibroblasts

The invasion of activated fibroblasts is a key mechanism of tissue fibrosis pathology. The recognition and uptake of apoptotic cells can induce the anti-fibrogenic programming of macrophages. We demonstrate that after interacting with apoptotic cells, macrophages secrete bioactive molecules that antagonize TGF-β1-induced increases in myofibroblast (fibroproliferative) phenotypic markers and reduce the enhanced invasive capacity of TGF-β1- or EGF-treated mouse lung fibroblasts (MLg). Furthermore, numerous treatment strategies prevented the anti-fibrotic effects of conditioned media, including transfection of macrophages with COX-2 or RhoA siRNAs or treatment of MLg cells with receptor antagonists for prostaglandin E₂ (PGE₂), PGD₂, or hepatocyte growth factor (HGF). Additionally, administration of apoptotic cells in vivo inhibited the bleomycin-mediated invasive capacity of primary fibroblasts, as well as adhesion and extracellular matrix protein mRNA expression. These data suggest that the anti-fibrogenic programming of macrophages by apoptotic cells can be used as a novel tool to control the progressive fibrotic reaction.

Harnessing Apoptotic Cell Clearance to Treat Autoimmune Arthritis

Early-stage apoptotic cells possess immunomodulatory properties. Proper apoptotic cell clearance during homeostasis has been shown to limit subsequent immune responses. Based on these observations, early-stage apoptotic cell infusion has been used to prevent unwanted inflammatory responses in different experimental models of autoimmune diseases or transplantation. Moreover, this approach has been shown to be feasible without any toxicity in patients undergoing allogeneic hematopoietic cell transplantation to prevent graft-versus-host disease. However, whether early-stage apoptotic cell infusion can be used to treat ongoing inflammatory disorders has not been reported extensively. Recently, we have provided evidence that early-stage apoptotic cell infusion is able to control, at least transiently, ongoing collagen-induced arthritis. This beneficial therapeutic effect is associated with the modulation of antigen-presenting cell functions mainly of macrophages and plasmacytoid dendritic cells, as well as the induction of collagen-specific regulatory CD4⁺ T cells (Treg). Furthermore, the efficacy of this approach is not altered by the association with two standard treatments of rheumatoid arthritis (RA), methotrexate and tumor necrosis factor (TNF) inhibition. Here, in the light of these observations and recent data of the literature, we discuss the mechanisms of early-stage apoptotic cell infusion and how this therapeutic approach can be transposed to patients with RA.

Selective IgM Deficiency-An Underestimated Primary Immunodeficiency

Although selective IgM deficiency (SIGMD) was described almost five decades ago, it was largely ignored as a primary immunodeficiency. SIGMD is defined as serum IgM levels below two SD of mean with normal serum IgG and IgA. It appears to be more common than originally realized. SIGMD is observed in both children and adults. Patients with SIGMD may be asymptomatic; however, approximately 80% of patients with SIGMD present with infections with bacteria, viruses, fungi, and protozoa. There is an increased frequency of allergic and autoimmune diseases in SIGMD. A number of B cell subset abnormalities have been reported and impaired specific antibodies to Streptococcus pneumoniae responses are observed in more than 45% of cases. Innate immunity, T cells, T cell subsets, and T cell functions are essentially normal. The pathogenesis of SIGMD remains unclear. Mice selectively deficient in secreted IgM are also unable to control infections from bacterial, viral, and fungal pathogens, and develop autoimmunity. Immunological and clinical similarities and differences between mouse models of deficiency of secreted IgM and humans with SIGMD have been discussed. Patients with SIGMD presenting with recurrent infections and specific antibody deficiency responses appear to improve clinically on immunoglobulin therapy.

Blimp-1-dependent and -independent natural antibody production by B-1 and B-1-derived plasma cells

Natural antibodies contribute to tissue homeostasis and protect against infections. They are secreted constitutively without external antigenic stimulation. The differentiation state and regulatory pathways that enable continuous natural antibody production by B-1 cells, the main cellular source in mice, remain incompletely understood. Here we demonstrate that natural IgM-secreting B-1 cells in the spleen and bone marrow are heterogeneous, consisting of (a) terminally differentiated B-1-derived plasma cells expressing the transcriptional regulator of differentiation, Blimp-1, (b) Blimp-1⁺, and (c) Blimp-1^neg phenotypic B-1 cells. Blimp-1^neg IgM-secreting B-1 cells are not simply intermediates of cellular differentiation. Instead, they secrete similar amounts of IgM in wild-type and Blimp-1-deficient (PRDM-1^ΔEx1A) mice. Blimp-1^neg B-1 cells are also a major source of IgG3. Consequently, deletion of Blimp-1 changes neither serum IgG3 levels nor the amount of IgG3 secreted per cell. Thus, the pool of natural antibody-secreting B-1 cells is heterogeneous and contains a distinct subset of cells that do not use Blimp-1 for initiation or maximal antibody secretion.

Small molecule-based inhibition of MEK1/2 proteins dampens inflammatory responses to malaria, reduces parasite load, and mitigates pathogenic outcomes

Malaria infections cause several systemic and severe single- or multi-organ pathologies, killing hundreds of thousands of people annually. Considering the existing widespread resistance of malaria parasites to anti-parasitic drugs and their high propensity to develop drug resistance, alternative strategies are required to manage malaria infections. Because malaria is a host immune response-driven disease, one approach is based on gaining a detailed understanding of the molecular and cellular processes that modulate malaria-induced innate and adaptive immune responses. Here, using a mouse cerebral malaria model and small-molecule inhibitors, we demonstrate that inhibiting MEK1/2, the upstream kinases of ERK1/2 signaling, alters multifactorial components of the innate and adaptive immune responses, controls parasitemia, and blocks pathogenesis. Specifically, MEK1/2 inhibitor treatment up-regulated B1 cell expansion, IgM production, phagocytic receptor expression, and phagocytic activity, enhancing parasite clearance by macrophages and neutrophils. Further, the MEK1/2 inhibitor treatment down-regulated pathogenic pro-inflammatory and helper T cell 1 (Th1) responses and up-regulated beneficial anti-inflammatory cytokine responses and Th2 responses. These inhibitor effects resulted in reduced granzyme B expression by T cells, chemokine and intracellular cell adhesion molecule 1 (ICAM-1) expression in the brain, and chemokine receptor expression by both myeloid and T cells. These bimodal effects of the MEK1/2 inhibitor treatment on immune responses contributed to decreased parasite biomass, organ inflammation, and immune cell recruitment, preventing tissue damage and death. In summary, we have identified several previously unrecognized immune regulatory processes through which a MEK1/2 inhibitor approach controls malaria parasitemia and mitigates pathogenic effects on host organs.

Detection of bacterial-reactive natural IgM antibodies in desert bighorn sheep populations

Ecoimmunology is a burgeoning field of ecology which studies immune responses in wildlife by utilizing general immune assays such as the detection of natural antibody. Unlike adaptive antibodies, natural antibodies are important in innate immune responses and often recognized conserved epitopes present in pathogens. Here, we describe a procedure for measuring natural antibodies reactive to bacterial antigens that may be applicable to a variety of organisms. IgM from desert bighorn sheep plasma samples was tested for reactivity to outer membrane proteins from Vibrio coralliilyticus, a marine bacterium to which sheep would have not been exposed. Immunoblotting demonstrated bighorn sheep IgM could bind to a variety of bacterial cell envelope proteins while ELISA analysis allowed for rapid determination of natural antibody levels in hundreds of individual animals. Natural antibody levels were correlated with the ability of plasma to kill laboratory strains of E. coli bacteria. Finally, we demonstrate that natural antibody levels varied in two distinct populations of desert bighorn sheep. These data demonstrate a novel and specific measure of natural antibody function and show that this varies in ecologically relevant ways.

The many ways tissue phagocytes respond to dying cells

Apoptosis is an important component of normal tissue physiology, and the prompt removal of apoptotic cells is equally essential to avoid the undesirable consequences of their accumulation and disintegration. Professional phagocytes are highly specialized for engulfing apoptotic cells. The recent ability to track cells that have undergone apoptosis in situ has revealed a division of labor among the tissue resident phagocytes that sample them. Macrophages are uniquely programmed to process internalized apoptotic cell-derived fatty acids, cholesterol and nucleotides, as a reflection of their dominant role in clearing the bulk of apoptotic cells. Dendritic cells carry apoptotic cells to lymph nodes where they signal the emergence and expansion of highly suppressive regulatory CD4 T cells. A broad suppression of inflammation is executed through distinct phagocyte-specific mechanisms. A clever induction of negative regulatory nodes is notable in dendritic cells serving to simultaneously shut down multiple pathways of inflammation. Several of the genes and pathways modulated in phagocytes in response to apoptotic cells have been linked to chronic inflammatory and autoimmune diseases such as atherosclerosis, inflammatory bowel disease and systemic lupus erythematosus. Our collective understanding of old and new phagocyte functions after apoptotic cell phagocytosis demonstrates the enormity of ways to mediate immune suppression and enforce tissue homeostasis.

DNA methylation governs the dynamic regulation of inflammation by apoptotic cells during efferocytosis

Efficient clearance of apoptotic cells (AC) is pivotal in preventing autoimmunity and is a potent immunosuppressive stimulus. However, activation of cells prior to apoptosis abolishes their immunoregulatory properties. Here we show using the antigen-induced model of arthritis that the degree of DNA methylation within AC confers their immunomodulatory plasticity. DNA isolated from resting and activated AC mimicked their respective immune effects. Demethylation of DNA abrogated the protective effect of AC whereas remethylation of AC DNA reversed the effects of activation and restored the ability to inhibit inflammation. Disease suppression or lack thereof was associated with TGFβ and IL-6 production respectively. Apoptotic CD4⁺ T cells from patients with rheumatoid arthritis and systemic lupus erythematosus were demethylated compared to healthy controls and favoured production of IL-6 when cultured with healthy macrophages, in contrast to the TGFβ produced in response to healthy AC. Our data implicate AC DNA methylation as the molecular switch that imprints their regulatory properties.

Peripheral blood mononuclear cell secretome for tissue repair

For almost two decades, cell-based therapies have been tested in modern regenerative medicine to either replace or regenerate human cells, tissues, or organs and restore normal function. Secreted paracrine factors are increasingly accepted to exert beneficial biological effects that promote tissue regeneration. These factors are called the cell secretome and include a variety of proteins, lipids, microRNAs, and extracellular vesicles, such as exosomes and microparticles. The stem cell secretome has most commonly been investigated in pre-clinical settings. However, a growing body of evidence indicates that other cell types, such as peripheral blood mononuclear cells (PBMCs), are capable of releasing significant amounts of biologically active paracrine factors that exert beneficial regenerative effects. The apoptotic PBMC secretome has been successfully used pre-clinically for the treatment of acute myocardial infarction, chronic heart failure, spinal cord injury, stroke, and wound healing. In this review we describe the benefits of choosing PBMCs instead of stem cells in regenerative medicine and characterize the factors released from apoptotic PBMCs. We also discuss pre-clinical studies with apoptotic cell-based therapies and regulatory issues that have to be considered when conducting clinical trials using cell secretome-based products. This should allow the reader to envision PBMC secretome-based therapies as alternatives to all other forms of cell-based therapies.

Germinal centers and autoimmune disease in humans and mice

Antibodies are involved in the pathogenesis of many autoimmune diseases. Although the mechanisms underlying the antibody response to infection or vaccination are reasonably well understood, we still have a poor understanding of the nature of autoimmune antibody responses. The most well studied are the anti-nuclear antibody responses characteristic of systemic lupus erythematosus and studies over the past decade or so have demonstrated a critical role for signaling by TLR7 and/or TLR9 in B cells to promote these responses. These Toll-like receptors (TLRs) can promote T-cell-independent extrafollicular antibody responses with a heavy-chain class switch and a low degree of somatic mutation, but they can also strongly boost the germinal center response that gives rise to high-affinity antibodies and long-lived plasma cells. TLRs have been shown to enhance affinity maturation in germinal center responses to produce high-affinity neutralizing antibodies in several virus infection models of mice. Although more data are needed, it appears that anti-nuclear antibodies in mouse models of lupus and in lupus patients can be generated by either pathway, provided there are genetic susceptibility alleles that compromise B-cell tolerance at one or another stage. Limited data in other autoimmune diseases suggest that the germinal center response may be the predominant pathway leading to autoantibodies in those diseases. A better understanding of the mechanisms of autoantibody production may ultimately be helpful in the development of targeted therapeutics for lupus or other autoimmune diseases.

Apoptotic cell infusion treats ongoing collagen-induced arthritis, even in the presence of methotrexate, and is synergic with anti-TNF therapy

Background

Apoptotic cell-based therapies have been proposed to treat chronic inflammatory diseases. The aim of this study was to investigate the effect of intravenous (i.v.) apoptotic cell infusion in ongoing collagen-induced arthritis (CIA) and the interaction of this therapy with other treatments used in rheumatoid arthritis (RA), including methotrexate (MTX) or anti-TNF therapy.

Methods

The effects of i.v. apoptotic cell infusion were evaluated in a CIA mouse model in DBA/1 mice immunized with bovine type II collagen. The number and functions of antigen-presenting cells (APC), regulatory CD4⁺ T cells (Treg), and circulating anti-collagen auto-antibodies were analyzed in CIA mice.

Results

Treatment of arthritic mice with i.v. apoptotic cell infusion significantly reduced the arthritis clinical score. This therapeutic approach modified T cell responses against the collagen auto-antigen with selective induction of collagen-specific Treg. In addition, we observed that APC from apoptotic-cell-treated animals were resistant to toll-like receptor ligand activation and favored ex vivo Treg induction, indicating APC reprogramming. Apoptotic cell injection-induced arthritis modulation was dependent on transforming growth factor (TGF)-β, as neutralizing anti-TGF-β antibody prevented the effects of apoptotic cells. Methotrexate did not interfere, while anti-TNF therapy was synergic with apoptotic-cell-based therapy.

Conclusion

Overall, our data demonstrate that apoptotic-cell-based therapy is efficient in treating ongoing CIA, compatible with current RA treatments, and needs to be evaluated in humans in the treatment of RA.

Malondialdehyde epitopes as mediators of sterile inflammation

Enhanced lipid peroxidation occurs during oxidative stress and results in the generation of lipid peroxidation end products such as malondialdehyde (MDA), which can attach to autologous biomolecules, thereby generating neo-self epitopes capable of inducing potentially undesired biological responses. Therefore, the immune system has developed mechanisms to protect from MDA epitopes by binding and neutralizing them through both cellular and soluble effectors. Here, we briefly discuss innate immune responses targeting MDA epitopes and their pro-inflammatory properties, followed by a review of physiological carriers of MDA epitopes that are relevant in homeostasis and disease. Then we discuss in detail the evidence for cellular responses towards MDA epitopes mainly in lung, liver and the circulation as well as signal transduction mechanisms and receptors implicated in the response to MDA epitopes. Last, we hypothesize on the role of MDA epitopes as mediators of inflammation in diseases and speculate on their contribution to disease pathogenesis. This article is part of a Special Issue entitled: Lipid modification and lipid peroxidation products in innate immunity and inflammation edited by Christoph J. Binder.

Concise Review: Apoptotic Cell-Based Therapies-Rationale, Preclinical Results and Future Clinical Developments

The objectives of this review are to summarize the experimental data obtained using apoptotic cell-based therapies, and then to discuss future clinical developments. Indeed, apoptotic cells exhibit immunomodulatory properties that are reviewed here by focusing on more recent mechanisms. These immunomodulatory mechanisms are in particular linked to the clearance of apoptotic cells (called also efferocytosis) by phagocytes, such as macrophages, and the induction of regulatory T cells. Thus, apoptotic cell-based therapies have been used to prevent or treat experimental inflammatory diseases. Based on these studies, we have identified critical steps to design future clinical trials. This includes: the administration route, the number and schedule of administration, the appropriate apoptotic cell type to be used, as well as the apoptotic signal. We also have analyzed the clinical relevancy of apoptotic-cell-based therapies in experimental models. Additional experimental data are required concerning the treatment of inflammatory diseases (excepted for sepsis) before considering future clinical trials. In contrast, apoptotic cells have been shown to favor engraftment and to reduce acute graft-versus-host disease (GvHD) in different relevant models of transplantation. This has led to the conduct of a phase 1/2a clinical trial to alleviate GvHD. The absence of toxic effects obtained in this trial may support the development of other clinical studies based on this new cell therapy. Stem Cells 2016;34:1464-1473.

Concise Review: Mechanisms Behind Apoptotic Cell-Based Therapies Against Transplant Rejection and Graft versus Host Disease

The main limitations to the success of transplantation are the antigraft response developed by the recipient immune system, and the adverse side effects of chronic immunosuppression. Graft-versus-host disease (GVHD) triggered by donor-derived T lymphocytes against the recipient tissues is another serious obstacle in the field of hematopoietic stem cell transplantation. Several laboratories have tested the possibility of promoting antigen (Ag)-specific tolerance for therapy of graft rejection, GVHD, and autoimmune disorders, by developing methodologies that mimic the mechanisms by which the immune system maintains peripheral tolerance in the steady state. It has been long recognized that the silent clearance of cells undergoing apoptosis exerts potent immune-regulatory effects and provides apoptotic cell-derived Ags to those Ag-presenting cells (APCs) that internalize them, in particular macrophages and dendritic cells. Therefore, in situ-targeting of recipient APCs by systemic administration of leukocytes in early apoptosis and bearing donor Ags represents a relatively simple approach to control the antidonor response against allografts. Here, we review the mechanisms by which apoptotic cells are silently cleared by phagocytes, and how such phenomenon leads to down-regulation of the innate and adaptive immunity. We discuss the evolution of apoptotic cell-based therapies from murine models of organ/tissue transplantation and GVHD, to clinical trials. We make emphasis on potential limitations and areas of concern of apoptotic cell-based therapies, and on how other immune-suppressive therapies used in the clinics or tested experimentally likely also function through the silent clearance of apoptotic cells by the immune system. Stem Cells 2016;34:1142-1150.

Autoantibodies in SLE: Specificities, Isotypes and Receptors

Systemic Lupus Erythematosus (SLE) is characterized by a wide spectrum of auto-antibodies which recognize several cellular components. The production of these self-reactive antibodies fluctuates during the course of the disease and the involvement of different antibody-secreting cell populations are considered highly relevant for the disease pathogenesis. These cells are developed and stimulated through different ways leading to the secretion of a variety of isotypes, affinities and idiotypes. Each of them has a particular mechanism of action binding to a specific antigen and recognized by distinct receptors. The effector responses triggered lead to a chronic tissue inflammation. DsDNA autoantibodies are the most studied as well as the first in being characterized for its pathogenic role in Lupus nephritis. However, others are of growing interest since they have been associated with other organ-specific damage, such as anti-NMDAR antibodies in neuropsychiatric clinical manifestations or anti-β2GP1 antibodies in vascular symptomatology. In this review, we describe the different auto-antibodies reported to be involved in SLE. How autoantibody isotypes and affinity-binding to their antigen might result in different pathogenic responses is also discussed.

Immunoglobulin M: Restrainer of Inflammation and Mediator of Immune Evasion by Plasmodium falciparum Malaria

Immunoglobulin M (IgM) is an ancient antibody class that is found in all vertebrates, with the exception of coelacanths, and is indispensable in both innate and adaptive immunity. The equally ancient human malaria parasite, Plasmodium falciparum, formed an intimate relationship with IgM with which it co-evolved. In this article, we discuss the association between IgM and human malaria parasites, building on several recent publications that implicate IgM as a crucial molecule that determines both host and parasite survival. Consequently, a better understanding of this association may lead to the development of improved intervention strategies.

Regulatory B cells mediate tolerance to apoptotic self in health: implications for disease

B cells are able to regulate immune responses through the secretion of IL-10 and other inhibitory cytokines, though no transcription factor that can define 'regulatory B cells' as a separate lineage has yet been found. Instead it is likely that this function arises as a result of the immune context in which B cells find themselves and the stimuli they perceive. However, some B cells found within the B1a and the marginal zone subsets have a greater propensity to produce IL-10 than others. What are the natural stimuli for these cells to induce immune regulation? We discuss the role that the recognition of autoantigens exposed by apoptotic cells plays in stimulating IL-10 production in mouse and human studies. This mechanism involves the recognition and uptake of self-antigens by autoreactive BCRs, for delivery to endocytic compartments, where apoptosis-derived DNA binds to TLR9, driving IL-10 production. These 'natural' regulatory B cells represent a way of maintaining tolerance to self. We discuss how this may operate in inflammatory lesions where there is an excess of apoptotic leukocytes and how this impacts on our understanding of autoimmune disease.

TIM-1 signaling is required for maintenance and induction of regulatory B cells

Apart from their role in humoral immunity, B cells can exhibit IL-10-dependent regulatory activity (Bregs). These regulatory subpopulations have been shown to inhibit inflammation and allograft rejection. However, our understanding of Bregs has been hampered by their rarity, lack of a specific marker, and poor insight into their induction and maintenance. We previously demonstrated that T cell immunoglobulin mucin domain-1 (TIM-1) identifies over 70% of IL-10-producing B cells, irrespective of other markers. We now show that TIM-1 is the primary receptor responsible for Breg induction by apoptotic cells (ACs). However, B cells that express a mutant form of TIM-1 lacking the mucin domain (TIM-1(Δmucin) ) exhibit decreased phosphatidylserine binding and are unable to produce IL-10 in response to ACs or by specific ligation with anti-TIM-1. TIM-1(Δmucin) mice also exhibit accelerated allograft rejection, which appears to be due in part to their defect in both baseline and induced IL-10(+) Bregs, since a single transfer of WT TIM-1(+) B cells can restore long-term graft survival. These data suggest that TIM-1 signaling plays a direct role in Breg maintenance and induction both under physiological conditions (in response to ACs) and in response to therapy through TIM-1 ligation. Moreover, they directly demonstrate that the mucin domain regulates TIM-1 signaling.