SOCS1 is a critical checkpoint in immune homeostasis, inflammation and tumor immunity

The Suppressor of Cytokine Signaling (SOCS) family proteins are important negative regulators of cytokine signaling. SOCS1 is the prototypical member of the SOCS family and functions in a classic negative-feedback loop to inhibit signaling in response to interferon, interleukin-12 and interleukin-2 family cytokines. These cytokines have a critical role in orchestrating our immune defence against viral pathogens and cancer. The ability of SOCS1 to limit cytokine signaling positions it as an important immune checkpoint, as evidenced by the detection of detrimental SOCS1 variants in patients with cytokine-driven inflammatory and autoimmune disease. SOCS1 has also emerged as a key checkpoint that restricts anti-tumor immunity, playing both a tumor intrinsic role and impacting the ability of various immune cells to mount an effective anti-tumor response. In this review, we describe the mechanism of SOCS1 action, focusing on the role of SOCS1 in autoimmunity and cancer, and discuss the potential for new SOCS1-directed cancer therapies that could be used to enhance adoptive immunotherapy and immune checkpoint blockade.

Severe persistent mycobacteria antigen stimulation causes lymphopenia through impairing hematopoiesis

Miliary tubersculosis (TB), an acute systemic blood disseminated tuberculosis mainly caused by Mycobacterium tuberculosis (M. tuberculosis), can cause signs of lymphopenia in clinical patients. To investigate whether/how persistent mycobacteria antigen stimulation impairs hematopoiesis and the therapeutic effect of interleukin-7 (IL-7), a mouse model of Mycobacterium Bovis Bacillus Calmette-Guérin (BCG) intravenous infection with/without an additional stimulation with M. tuberculosis multi-antigen cocktail containing ESAT6-CFP10 (EC) and Mtb10.4-HspX (MH) was established. Consistent with what happened in miliary TB, high dose of BCG intravenous infection with/without additional antigen stimulation caused lymphopenia in peripheral blood. In which, the levels of cytokines IFN-γ and TNF-α in serum increased, and consequently the expression levels of transcription factors Batf2 and IRF8 involved in myeloid differentiation were up-regulated, while the expression levels of transcription factors GATA2 and NOTCH1 involved in lymphoid commitment were down-regulated, and the proliferating activity of bone marrow (BM) lineage^- c-Kit⁺ (LK) cells decreased. Furthermore, recombinant Adeno-Associated Virus 2-mediated IL-7 (rAAV2-IL-7) treatment could significantly promote the elevation of BM lymphoid progenitors. It suggests that persistent mycobacteria antigen stimulation impaired lymphopoiesis of BM hematopoiesis, which could be restored by complement of IL-7.

Signaling networks in B cell development and related therapeutic strategies

B cells are essential for Ab production during humoral immune responses. From decades of B cell research, there is now a detailed understanding of B cell subsets, development, functions, and most importantly, signaling pathways. The complicated pathways in B cells and their interactions with each other are stage-dependent, varying with surface marker expression during B cell development. With the increasing understanding of B cell development and signaling pathways, the mechanisms underlying B cell related diseases are being unraveled as well, making it possible to provide more precise and effective treatments. In this review, we describe several essential and recently discovered signaling pathways in B cell development and take a look at newly developed therapeutic strategies targeted at B cell signaling.

Cytokine levels of interleukin-2 and 7 amongst antiretroviral therapy success and failure HIV patients attending the University Teaching Hospital, Yaoundé, Cameroon

BACKGROUND

Immune reconstitution complications (IRC) are a major problem faced by HIV treated patients world wide. Interleukin (IL)-2 and IL-7 play vital roles in peripheral T-cell homeostasis. Our study objective was to measure and compare the blood plasma levels of IL-2 and IL-7 amongst antiretroviral therapy (ART) patients attending the Yaoundé University Teaching Hospital, Cameroon.

METHODS

We performed a cross-sectional study with 296 HIV positive patients enrolled between July 2017 and May 2018 at the Yaoundé University Teaching Hospital. IL-2, IL-7, T-cell profile counts and plasma viral load were measured on whole blood specimens. Data obtained were analyzed using Graph Pad Prism 5.0 and Epi info 7.0. Software.

RESULTS

IL-2 and IL-7 plasma concentration levels were higher in patients with ART failure compared to ART success, with a mean SD of 19.4±8 and 17.1±6 pg /ml, 35.26±11 and 21.5±5 pg/ml, with p < 0.001 and < 0.001. There was a direct and significant correlation between viral load, IL-2 and IL-7 with p values = 0.028, and 0.020, respectively. There was an association between IL-2, IL-7 and viral load in relation to the duration on treatment (DT), with p values = 0.003 (R²=0.041, CI= 0.069 - 0.34) ,0.017 (R²=0.027, CI=-0.30 - 0.030), and 0.001 (R²=0.048, CI=-0.047-0.76).

CONCLUSION

Considering that limited surrogate markers are availiable for monitoring immune reconstitution and high associated mortality rates, IL-2 and IL-7 could be a good immunological predictor for ART failure and success in HIV infected individuals.

The Regulatory Role of IFN-γ on the Proliferation and Differentiation of Hematopoietic Stem and Progenitor Cells

The replenishment of all blood cell lineages is hierarchically organized by the process of hematopoiesis, which is based on the differentiation pathways of hematopoietic stem and progenitor cells (HSPCs). Due to the ability to balance between self-renewal and differentiation, hematopoietic stem cells (HSCs) can generate the appropriate cell type that is required by the immune system and peripheral blood in response to physiological or pathological conditions. Numerous studies have shown that some proinflammatory cytokines contribute to the regulation of the various hematopoietic compartments. Of these, IFN-γ is a type II interferon primarily produced by T cells and natural killer cells, and plays a major role in the defense against invading pathogens and transformed cancer cells; moreover, a growing amount of research indicates that it exerts negative or positive regulatory effect on hematopoiesis. Although IFN-γ is a widely regarded negative regulator of HSC proliferation, it also participates in some chronic infections or hematological malignancies that induce bone marrow failure. Recent studies have demonstrated unexpected effects of IFN-γ, including the promotion of HSC formation and the stimulation of myelopoiesis. Here, we review the direct and indirect effects of IFN-γ on hematopoiesis, as well as the underlying signaling mechanisms of how IFN-γ modulates the self-renewal, cell cycle entry, and proliferation of HSCs. Next, we describe how IFN-γ affects different stages of the lineage differentiation from HSCs. Finally, we discuss the relationship between IFN-γ and compensatory extramedullary hematopoiesis, as well as some related clinical diseases.

Myeloid-Derived Suppressor Cells Impair B Cell Responses in Lung Cancer through IL-7 and STAT5

Myeloid-derived suppressor cells (MDSCs) are known suppressors of antitumor immunity, affecting amino acid metabolism and T cell function in the tumor microenvironment. However, it is unknown whether MDSCs regulate B cell responses during tumor progression. Using a syngeneic mouse model of lung cancer, we show reduction in percentages and absolute numbers of B cell subsets including pro-, pre-, and mature B cells in the bone marrow (BM) of tumor-bearing mice. The kinetics of this impaired B cell response correlated with the progressive infiltration of MDSCs. We identified that IL-7 and downstream STAT5 signaling that play a critical role in B cell development and differentiation were also impaired during tumor progression. Global impairment of B cell function was indicated by reduced serum IgG levels. Importantly, we show that anti-Gr-1 Ab-mediated depletion of MDSCs not only rescued serum IgG and IL-7 levels but also reduced TGF-β1, a known regulator of stromal IL-7, suggesting MDSC-mediated regulation of B cell responses. Furthermore, blockade of IL-7 resulted in reduced phosphorylation of downstream STAT5 and B cell differentiation in tumor-bearing mice and administration of TGF-β-blocking Ab rescued these IL-7-dependent B cell responses. Adoptive transfer of BM-derived MDSCs from tumor-bearing mice into congenic recipients resulted in significant reductions of B cell subsets in the BM and in circulation. MDSCs also suppressed B cell proliferation in vitro in an arginase-dependent manner that required cell-to-cell contact. Our results indicate that tumor-infiltrating MDSCs may suppress humoral immune responses and promote tumor escape from immune surveillance.

c-Myb Coordinates Survival and the Expression of Genes That Are Critical for the Pre-BCR Checkpoint

The c-Myb transcription factor is required for adult hematopoiesis, yet little is known about c-Myb function during lineage-specific differentiation due to the embryonic lethality of Myb-null mutations. We previously used tissue-specific inactivation of the murine Myb locus to demonstrate that c-Myb is required for differentiation to the pro-B cell stage, survival during the pro-B cell stage, and the pro-B to pre-B cell transition during B lymphopoiesis. However, few downstream mediators of c-Myb-regulated function have been identified. We demonstrate that c-Myb regulates the intrinsic survival of CD19⁺ pro-B cells in the absence of IL-7 by repressing expression of the proapoptotic proteins Bmf and Bim and that levels of Bmf and Bim mRNA are further repressed by IL-7 signaling in pro-B cells. c-Myb regulates two crucial components of the IL-7 signaling pathway: the IL-7Rα-chain and the negative regulator SOCS3 in CD19⁺ pro-B cells. Bypassing IL-7R signaling through constitutive activation of Stat5b largely rescues survival of c-Myb-deficient pro-B cells, whereas constitutively active Akt is much less effective. However, rescue of pro-B cell survival is not sufficient to rescue proliferation of pro-B cells or the pro-B to small pre-B cell transition, and we further demonstrate that c-Myb-deficient large pre-B cells are hypoproliferative. Analysis of genes crucial for the pre-BCR checkpoint demonstrates that, in addition to IL-7Rα, the genes encoding λ5, cyclin D3, and CXCR4 are downregulated in the absence of c-Myb, and λ5 is a direct c-Myb target. Thus, c-Myb coordinates survival with the expression of genes that are required during the pre-BCR checkpoint.

IFN-γ directly inhibits murine B-cell precursor leukemia-initiating cell proliferation early in life

The early-life immune environment has been implicated as a modulator of acute lymphoblastic leukemia (ALL) development in children, with infection being associated with significant changes in ALL risk. Furthermore, polymorphisms in several cytokine genes, including IL-10 and IFN-γ, are associated with leukemia development. However, the mechanisms and timing of these influences remain unknown. Here, we use the Eμ-ret transgenic mouse model of B-cell precursor ALL to assess the influence of IFN-γ on the early-life burden of leukemia-initiating cells. The absence of IFN-γ activity resulted in greater numbers of leukemia-initiating cells early in life and was associated with accelerated leukemia onset. The leukemia-initiating cells from IFN-γ-knockout mice had reduced suppressor of cytokine signaling (SOCS-1) expression, were significantly more sensitive to IFN-γ, and exhibited more rapid expansion in vivo than their wild-type counterparts. However, sensitivity to this inhibitory pathway was lost in fully transformed IFN-γ-knockout leukemia cells. These results demonstrate that the influence of IFN-γ on ALL progression may not be mediated by selection of nascent transformed cells but rather through a general SOCS-mediated reduction in B-cell precursor proliferation. Thus, while cytokine levels may influence leukemia at multiple points during disease progression, our study indicates a significant early influence of basal, infection-independent cytokine production on leukemogenesis.

Interleukin-7 and Immunosenescence

The age of an individual is an important, independent risk factor for many of the most common diseases afflicting modern societies. Interleukin-7 (IL-7) plays a central, critical role in the homeostasis of the immune system. Recent studies support a critical role for IL-7 in the maintenance of a vigorous healthspan. We describe the role of IL-7 and its receptor in immunosenescence, the aging of the immune system. An understanding of the role that IL-7 plays in aging may permit parsimonious preventative or therapeutic solutions for diverse conditions. Perhaps IL-7 might be used to "tune" the immune system to optimize human healthspan and longevity.

Suppressor of cytokine signaling (SOCS) proteins are induced by IL-7 and target surface CD127 protein for degradation in human CD8 T cells

Given the essential role interleukin (IL)-7 plays in T-cell survival, homeostasis and function, it is no surprise expression of the IL-7 receptor alpha-chain (CD127) is tightly regulated. We have previously shown IL-7 binding to its receptor on the surface of CD8 T cells leads to both suppression of CD127 gene transcription and loss of existing CD127 protein from the cell membrane. Indeed upon binding IL-7, CD127 is rapidly internalized into early endosomes where phosphorylation by JAK targets the receptor for degradation. We now show that IL-7 induces the expression of suppressor of cytokine signaling (SOCS) proteins CIS, SOCS1 and SOCS2 through the JAK/STAT-5 pathway and that CIS and SOCS2 specifically interact with CD127 in early endosomes and direct the receptor complex to the proteasome for degradation. These results illustrate how expression of the IL-7 receptor and thus IL-7 signaling is modulated in human CD8 T cells by a negative feedback mechanism dependent on members of the SOCS family of proteins.

Parallels between immune driven-hematopoiesis and T cell activation: 3 signals that relay inflammatory stress to the bone marrow

Quiescence, self-renewal, lineage commitment and differentiation of hematopoietic stem cells (HSCs) towards fully mature blood cells are a complex process that involves both intrinsic and extrinsic signals. During steady-state conditions, most hematopoietic signals are provided by various resident cells inside the bone marrow (BM), which establish the HSC micro-environment. However, upon infection, the hematopoietic process is also affected by pathogens and activated immune cells, which illustrates an effective feedback mechanism to hematopoietic stem and progenitor cells (HSPCs) via immune-mediated signals. Here, we review the impact of pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), costimulatory molecules and pro-inflammatory cytokines on the quiescence, proliferation and differentiation of HSCs and more committed progenitors. As modulation of HSPC function via these immune-mediated signals holds an interesting parallel with the "three-signal-model" described for the activation and differentiation of naïve T-cells, we propose a novel "three-signal" concept for immune-driven hematopoiesis. In this model, the recognition of PAMPs and DAMPs will activate HSCs and induce proliferation, while costimulatory molecules and pro-inflammatory cytokines confer a second and third signal, respectively, which further regulate expansion, lineage commitment and differentiation of HSPCs. We review the impact of inflammatory stress on hematopoiesis along these three signals and we discuss whether they act independently from each other or that concurrence of these signals is important for an adequate response of HSPCs upon infection.

Impact of interferon-γ on hematopoiesis

The proinflammatory cytokine interferon-γ (IFN-γ) is well known for its important role in innate and adaptive immunity against intracellular infections and for tumor control. Yet, it has become clear that IFN-γ also has a strong impact on bone marrow (BM) output during inflammation, as it affects the differentiation of most hematopoietic progenitor cells. Here, we review the impact of IFN-γ on hematopoiesis, including the function of hematopoietic stem cells (HSCs) and more downstream progenitors. We discuss which hematopoietic lineages are functionally modulated by IFN-γ and through which underlying molecular mechanism(s). We propose the novel concept that IFN-γ acts through upregulation of suppressor of cytokine signaling molecules, which impairs signaling of several cytokine receptors. IFN-γ has also gained clinical interest from different angles, and we discuss how chronic IFN-γ production can lead to the development of anemia and BM failure and how it is involved in malignant hematopoiesis. Overall, this review illustrates the wide-ranging effect of IFN-γ on the (patho-)physiological processes in the BM.

Toll-like receptor 4-mediated signaling regulates IL-7-driven proliferation and differentiation of B-cell precursors

Lipopolysaccharide (LPS) is known to be a potent activator of mature B cells by signaling through Toll-like receptor 4 (TLR4). Its impact on early B-cell development, however, is not well defined. When comparing to C3H/HeN mice, TLR4-mutant C3H/HeJ mice showed an increase in the number of pro-B and pre-B cells in the bone marrow. When cultured in the presence of IL-7, the proliferation of pro-B and large pre-B cells was significantly inhibited by LPS, possibly due to reduced IL-7 receptor-α (IL-7Rα) expression. Meanwhile, the generation of IgM(+)/IgD(+) B cells was greatly enhanced in IL-7 cultures of pro-B and pre-B cells. Consistent with these results, treatment with LPS facilitated the progression of adoptively transferred B220(+)IgM(-)IgD(-) precursors into IgD(+) cells. Overall, these data suggest that LPS has a profound influence on early B-cell development, which may contribute to the deregulated B-cell development under physiological and pathological conditions such as bacterial infections.

Interferon-γ impairs proliferation of hematopoietic stem cells in mice

Balancing the processes of hematopoietic stem cell (HSC) differentiation and self-renewal is critical for maintaining a lifelong supply of blood cells. The bone marrow (BM) produces a stable output of newly generated cells, but immunologic stress conditions inducing leukopenia increase the demand for peripheral blood cell supply. Here we demonstrate that the proinflammatory cytokine interferon-γ (IFN-γ) impairs maintenance of HSCs by directly reducing their proliferative capacity and that IFN-γ impairs restoration of HSC numbers upon viral infection. We show that IFN-γ reduces thrombopoietin (TPO)-mediated phosphorylation of signal transducer and activator of transcription (STAT) 5, an important positive regulator of HSC self-renewal. IFN-γ also induced expression of suppressor of cytokine signaling (SOCS) 1 in HSCs, and we demonstrate that SOCS1 expression is sufficient to inhibit TPO-induced STAT5 phosphorylation. Furthermore, IFN-γ deregulates expression of STAT5-mediated cell-cycle genes cyclin D1 and p57. These findings suggest that IFN-γ is a negative modulator of HSC self-renewal by modifying cytokine responses and expression of genes involved in HSC proliferation. We postulate that the occurrence of BM failure in chronic inflammatory conditions, such as aplastic anemia, HIV, and graft-versus-host disease, is related to a sustained impairment of HSC self-renewal caused by chronic IFN-γ signaling in these disorders.

Cutting edge: cell-autonomous control of IL-7 response revealed in a novel stage of precursor B cells

During early stages of B-lineage differentiation in bone marrow, signals emanating from IL-7R and pre-BCR are thought to synergistically induce proliferative expansion of progenitor cells. Paradoxically, loss of pre-BCR-signaling components is associated with leukemia in both mice and humans. Exactly how progenitor B cells perform the task of balancing proliferative burst dependent on IL-7 with the termination of IL-7 signals and the initiation of L chain gene rearrangement remains to be elucidated. In this article, we provide genetic and functional evidence that the cessation of the IL-7 response of pre-B cells is controlled via a cell-autonomous mechanism that operates at a discrete developmental transition inside Fraction C' (large pre-BII) marked by transient expression of c-Myc. Our data indicate that pre-BCR cooperates with IL-7R in expanding the pre-B cell pool, but it is also critical to control the differentiation program shutting off the c-Myc gene in large pre-B cells.