LAG-3 is not responsible for selecting T helper cells in CD4-deficient mice

LAG-3-An incompletely understood target in cancer therapy

LAG-3 is a member of the immunoglobulin superfamily expressed on activated T cells, but also on other immune cells. It has significant homology to CD4. Both molecules have four extracellular Ig-like domains with similar structural motifs but the sequence identity between LAG-3 and CD4 is low. Furthermore, unlike CD4 LAG-3 restrains T cell responses and antibodies targeting this receptor are emerging drugs in cancer immunotherapy. A combination of LAG-3 and PD-1 antibodies has already been approved for the treatment of metastatic melanoma. Despite this success, its biology is still not well understood. Here we summarize the current knowledge on expression, ligands, and function of LAG-3. We point to the differences between LAG-3 and other inhibitory immune checkpoints and describe obstacles to study the role of this receptor in T cell activation processes. Finally, we discuss future directions for scientific efforts to come to a more complete understanding of the biology of this eminent immune checkpoint.

Next generation of immune checkpoint molecules in maternal-fetal immunity

Successful pregnancy is a unique situation requires the maternal immune system to recognize and tolerate a semi-identical fetus and allow normal invasion of trophoblast cells. Although efforts have been made, the deep mechanisms of the maternal-fetal crosstalk have not yet been fully deciphered. Immune checkpoint molecules (ICMs) are a group of negative modulators of the immune response that avoid immune damage. They have been extensively studied in the fields of oncology and transplantation, while the latest evidence suggests that they are closely associated with pregnancy outcomes via multiple inhibitory mechanisms. Although studies have mostly demonstrated the regulatory role of the well-known PD-1, CTLA-4 at the maternal-fetal interface, what is unique about the newly discovered multiple ICMs remains a mystery. Here, we review the latest knowledge on ICMs, focusing on the first generation of checkpoints (PD-1, CTLA-4) and the next generation (Tim-3, Tigit, Lag-3, VISTA) highlighting their immunoregulatory roles in maternal-fetal tolerance and decidual vascular remodeling, and their involvement in pathological pregnancies. The content covers three aspects: the characteristics they possess, the dynamic expression profile of their expression at the maternal-fetal interface, and their involvement in pathological pregnancy. In immunotherapy strategies for pregnancy complications, upregulation of immune checkpoints may play a role. Meanwhile, the impact on pregnancy outcomes when using ICMs in clinical cancer treatment during pregnancy is a topic worth exploring. These may serve as a guide for future basic research and clinical applications of maternal-fetal immunity.

Natural killer cells in liver transplantation: Can we harness the power of the immune checkpoint to promote tolerance?

The roles that natural killer (NK) cells play in liver disease and transplantation remain ill‐defined. Reports on the matter are often contradictory, and the mechanisms elucidated are complex and dependent on the context of the model tested. Moreover, NK cell attributes, such as receptor protein expression and function differ among species, make study of primate or rodent transplant models challenging. Recent insights into NK function and NK‐mediated therapy in the context of cancer therapy may prove applicable to transplantation. Of specific interest are immune checkpoint molecules and the mechanisms by which they modulate NK cells in the tumor micro‐environment. In this review, we summarize NK cell populations in the peripheral blood and liver, and we explore the data regarding the expression and function of immune checkpoint molecules on NK cells. We also hypothesize about the roles they could play in liver transplantation and discuss how they might be harnessed therapeutically in transplant sciences.

Advanced Nanotechnology for Enhancing Immune Checkpoint Blockade Therapy

Immune checkpoint receptor signaling pathways constitute a prominent class of "immune synapse," a cell-to-cell connection that represses T-lymphocyte effector functions. As a possible evolutionary countermeasure against autoimmunity, this strategy is aimed at lowering potential injury to uninfected cells in infected tissues and at minimizing systemic inflammation. Nevertheless, tumors can make use of these strategies to escape immune recognition, and consequently, such mechanisms represent chances for immunotherapy intervention. Recent years have witnessed the advance of pharmaceutical nanotechnology, or nanomedicine, as a possible strategy to ameliorate immunotherapy technical weaknesses thanks to its intrinsic biophysical properties and multifunctional modifying capability. To improve the long-lasting response rate of checkpoint blockade therapy, nanotechnology has been employed at first for the delivery of single checkpoint inhibitors. Further, while therapy via single immune checkpoint blockade determines resistance and a restricted period of response, strong interest has been raised to efficiently deliver immunomodulators targeting different inhibitory pathways or both inhibitory and costimulatory pathways. In this review, the partially explored promise in implementation of nanotechnology to improve the success of immune checkpoint therapy and solve the limitations of single immune checkpoint inhibitors is debated. We first present the fundamental elements of the immune checkpoint pathways and then outline recent promising results of immune checkpoint blockade therapy in combination with nanotechnology delivery systems.

LAG-3: from molecular functions to clinical applications

To prevent the destruction of tissues owing to excessive and/or inappropriate immune responses, immune cells are under strict check by various regulatory mechanisms at multiple points. Inhibitory coreceptors, including programmed cell death 1 (PD-1) and cytotoxic T lymphocyte antigen 4 (CTLA-4), serve as critical checkpoints in restricting immune responses against self-tissues and tumor cells. Immune checkpoint inhibitors that block PD-1 and CTLA-4 pathways significantly improved the outcomes of patients with diverse cancer types and have revolutionized cancer treatment. However, response rates to such therapies are rather limited, and immune-related adverse events are also observed in a substantial patient population, leading to the urgent need for novel therapeutics with higher efficacy and lower toxicity. In addition to PD-1 and CTLA-4, a variety of stimulatory and inhibitory coreceptors are involved in the regulation of T cell activation. Such coreceptors are listed as potential drug targets, and the competition to develop novel immunotherapies targeting these coreceptors has been very fierce. Among such coreceptors, lymphocyte activation gene-3 (LAG-3) is expected as the foremost target next to PD-1 in the development of cancer therapy, and multiple clinical trials testing the efficacy of LAG-3-targeted therapy are underway. LAG-3 is a type I transmembrane protein with structural similarities to CD4. Accumulating evidence indicates that LAG-3 is an inhibitory coreceptor and plays pivotal roles in autoimmunity, tumor immunity, and anti-infection immunity. In this review, we summarize the current understanding of LAG-3, ranging from its discovery to clinical application.

Human NK cells: surface receptors, inhibitory checkpoints, and translational applications

NK cells play important roles in innate defenses against viruses and in the control of tumor growth and metastasis. The regulation/induction of NK cell function is mediated by an array of activating or inhibitory surface receptors. In humans, major activating receptors involved in target cell killing are the natural cytotoxicity receptors (NCRs) and NKG2D. Activating receptors recognize ligands that are overexpressed or expressed de novo upon cell stress, viral infection, or tumor transformation. The HLA-class I-specific inhibitory receptors, including KIRs recognizing HLA-class I allotypic determinants and CD94/NKG2A recognizing the class-Ib HLA-E, constitute a fail-safe mechanism to avoid unwanted NK-mediated damage to healthy cells. Other receptors such as PD-1, primarily expressed by activated T lymphocytes, are important inhibitory checkpoints of immune responses that ensure T-cell tolerance. PD-1 also may be expressed by NK cells in cancer patients. Since PD-1 ligand (PD-L1) may be expressed by different tumors, PD-1/PD-L1 interactions inactivate both T and NK cells. Thus, the reliable evaluation of PD-L1 expression in tumors has become a major issue to select patients who may benefit from therapy with mAbs disrupting PD-1/PD-L1 interactions. Recently, NKG2A was revealed to be an important checkpoint controlling both NK and T-cell activation. Since most tumors express HLA-E, mAbs targeting NKG2A has been used alone or in combination with other therapeutic mAbs targeting PD-1 or tumor antigens (e.g., EGFR), with encouraging results. The translational value of NK cells and their receptors is evidenced by the extraordinary therapeutic success of haploidentical HSCT to cure otherwise fatal high-risk leukemias.

Innate Lymphoid Cells: Expression of PD-1 and Other Checkpoints in Normal and Pathological Conditions

Innate lymphoid cells (ILCs) belong to a family of immune cells. Recently, ILCs have been classified into five different groups that mirror the function of adaptive T cell subsets counterparts. In particular, NK cells mirror CD8⁺ cytotoxic T cells while ILC1, ILC2, ILC3, and Lymphoid tissue inducer (LTi)-like cells reflect the function of CD4⁺T helper (Th) cells (Th1, Th2, and Th17 respectively). ILCs are involved in innate host defenses against pathogens and tumors, in lymphoid organogenesis, and in tissue remodeling/repair. In recent years, important molecular inducible checkpoints (PD-1, TIM3, and TIGIT) were shown to control/inactivate different immune cell types. The expression of many of these receptors has been detected on NK cells and subsets of tissue-resident ILCs in both physiological and pathological conditions, including cancer. In particular, it has been demonstrated that the interaction between PD-1⁺ immune cells and PD-L1/PD-L2+ tumor cells may compromise the anti-tumor effector function leading to tumor immune escape. However, while the effector function of NK cells in tumor is well-established, limited information exists on the other ILC subsets. We will summarize what is known to date on the expression and function of these checkpoint receptors on NK cells and ILCs, with a particular focus on the recent data that reveal an essential contribution of the blockade of PD-1 and TIGIT on NK cells to the immunotherapy of cancer. A better information regarding the presence and the function of different ILCs and of the inhibitory checkpoints in pathological conditions may offer important clues for the development of new immune therapeutic strategies.

Mechanistic overview of immune checkpoints to support the rational design of their combinations in cancer immunotherapy

Checkpoint receptor blockers, known to act by blocking the pathways that inhibit immune cell activation and stimulate immune responses against tumor cells, have been immensely successful in the treatment of cancer. Among several checkpoint receptors of immune cells, cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), programmed cell death protein-1 (PD-1), T-cell immunoglobulin and ITIM domain (TIGIT), T-cell immunoglobulin-3 (TIM-3) and lymphocyte activation gene 3 (LAG-3) are the most commonly targeted checkpoints for cancer immunotherapy. Six drugs including one CTLA-4 blocker (ipilimumab), two PD-1 blockers (nivolumab and pembrolizumab) and three PD-L1 blockers (atezolizumab, avelumab and durvalumab) are approved for the treatment of different types of cancers including both solid tumors such as melanoma, lung cancer, head and neck cancer, bladder cancer and Merkel cell cancer as well as hematological tumors such as classic Hodgkin's lymphoma. The main problem with checkpoint blockers is that only a fraction of patients respond to the therapy. Insufficient immune activation is considered as one of the main reason for low response rates and combination of checkpoint blockers has been proposed to increase the response rates. The combination of checkpoint blockers was successful in melanoma but had significant adverse events. A combination that is selected based on the mechanistic differences between checkpoints and the differences in expression of checkpoints and their ligands in the tumor microenvironment could have a synergistic effect in a given cancer subtype and also have a manageable safety profile. This review aims to help in design of optimal checkpoint blocker combinations by discussing the mechanistic details and outlining the subtle differences between major checkpoints targeted for cancer immunotherapy.

Monoclonal Antibodies for the Treatment of Melanoma: Present and Future Strategies

Metastatic melanoma is a dreadful type of skin cancer arising due to uncontrolled proliferation of melanocytes. It has very poor prognosis, low 5-year survival rates and until recently there were only handful of treatment options for metastatic melanoma patients. The drugs that were approved for the treatment had low response rates and were associated with severe adverse events. With the introduction of monoclonal antibodies against inhibitory immune checkpoints the treatment landscape for metastatic melanoma has changed dramatically. Ipilimumab, the first monoclonal antibody to be approved for the treatment of metastatic melanoma, showed significant improvements in durable response rates in patients and paved the way for next class of monoclonal antibodies. Nivolumab and pembrolizumab, the anti-PD-1 antibodies that were approved 3-years after the approval of ipilimumab, had decent response rates, low relapse rates and showed manageable safety profile. Antibodies against ligands for PD-1 receptors were then developed to overcome the adverse effects of anti-PD-1 antibodies and combination of monoclonal antibodies (ipilimumab plus nivolumab) was tested to increase the response rates. Additional target receptors that regulate T cell activity were identified on T cells and monoclonal antibodies against potential targets such as TIGIT, TIM-3, and LAG-3 were developed. This chapter discusses the details of monoclonal antibodies used for the treatment of melanoma along with the ones that could be introduced in the near future with emphasis on mechanisms by which antibodies stimulate anti-tumor immune response and the specifics of target molecules of the antibodies.

Identification of LAG3 high affinity aptamers by HT-SELEX and Conserved Motif Accumulation (CMA)

LAG3 receptor belongs to a family of immune-checkpoints expressed in T lymphocytes and other cells of the immune system. It plays an important role as a rheostat of the immune response. Focus on this receptor as a potential therapeutic target in cancer immunotherapy has been underscored after the success of other immune-checkpoint blockade strategies in clinical trials. LAG3 showcases the interest in the field of autoimmunity as several studies show that LAG3-targeting antibodies can also be used for the treatment of autoimmune diseases. In this work we describe the identification of a high-affinity LAG3 aptamer by High Throughput Sequencing SELEX in combination with a study of potential conserved binding modes according to sequence conservation by using 2D-structure prediction and 3D-RNA modeling using Rosetta. The aptamer with the highest accumulation of these conserved sequence motifs displays the highest affinity to LAG3 recombinant soluble proteins and binds to LAG3-expressing lymphocytes. The aptamer described herein has the potential to be used as a therapeutic agent, as it enhances the threshold of T-cell activation. Nonetheless, in future applications, it could also be engineered for treatment of autoimmune diseases by target depletion of LAG3-effector T lymphocytes.

Immunotherapy of cancers comes of age

INTRODUCTION

Cancer immunotherapy has evolved and is aimed at generating the efficacious therapeutic modality to enhance the specificity and power of the immune system to combat tumors. Areas covered: Current efforts in cancer immunotherapy fall into three main approaches. One approach is through the blockade of immune checkpoints, another approach is through adoptive cellular therapy, and the last approach is through vaccination. The goal of this review is to summarize the current understanding and status of cancer immunotherapy in these three categories. Expert commentary: We foresee the development of therapeutic protocols combining these approaches with each other or conventional therapies to achieve the most appropriate guideline for management of cancer.

LAG3 and PD1 co-inhibitory molecules collaborate to limit CD8+ T cell signaling and dampen antitumor immunity in a murine ovarian cancer model

The immune co-inhibitory receptors lymphocyte activation gene-3 (LAG3) and programmed cell death 1 (PD1) synergistically contribute to autoimmunity and tumor evasion. Here we demonstrate how they collaborate and interact to regulate T cell function. We first show that LAG3 and PD1 are co-expressed on both OVA-specific and non-specific T cells infiltrating murine ovarian tumors. Dual antibody blockade or genetic knockout of LAG3 and PD1 significantly enhanced T effector function and delayed tumor growth. LAG3 and PD1 co-localized in activated CD8⁺ T cells in vitro at the trans-Golgi vesicles, early/recycling endosomal compartments, lysosomes, and microtubule organizing center. Importantly, LAG3 and PD1 cluster with pLck at the immunological synapse. Reciprocal immunoprecipitation of T cell extracts revealed physical interaction between LAG3 and PD1. Mutational analyses indicate that the cytoplasmic domain of LAG3 is not absolutely required for its association with PD1, while the ITIM and ITSM of PD1 are necessary for its association with LAG3. Finally, LAG3 protein also associates with the Src-homology-2 domain-containing phosphatases (SHP1/2) which are known to be recruited by PD1 during T cell signaling. Our data indicate that the association of LAG3 with PD1 contributes to their rapid trafficking to the immunological synapse, leading to a synergistic inhibitory effect on T cell signaling.

Innovative perspectives of immunotherapy in head and neck cancer. From relevant scientific rationale to effective clinical practice

It is now well established that head and neck cancer carcinogenesis is characterized by genetic instability and several immune defects, leading to unique host-tumor interactions. In such condition, recent improved comprehension and relevant findings could lead to identification of innovative molecular therapeutic targets, achieving considerable clinical and translational research. This review aims to summarize and to highlight most recent and relevant scientific rationale in this era of immunotherapy revival, and to correlate it to the near future clinical practice for the management of this challenging disease.

Combinatorial Cancer Immunotherapies

T cell checkpoint blockade therapies are revolutionizing the treatment of patients with cancer. Highlighted by the recent success of PD-1 plus CTLA-4 blockade in patients with melanomas, synergistic immunotherapy combinations of modalities represent an important opportunity to improve responses and outcomes for patients. We review the rationale and experience with T cell checkpoint blockade in combination with targeting of other coinhibitory or costimulatory checkpoints, immunomodulatory molecules in the tumor microenvironment, and other anticancer modalities such as vaccines, chemotherapy, and radiation.

Negative immune checkpoints on T lymphocytes and their relevance to cancer immunotherapy

The term 'inhibitory checkpoint' refers to the broad spectrum of co-receptors expressed by T cells that negatively regulate T cell activation thus playing a crucial role in maintaining peripheral self-tolerance. Co-inhibitory receptor ligands are highly expressed by a variety of malignancies allowing evasion of anti-tumour immunity. Recent studies demonstrate that manipulation of these co-inhibitory pathways can remove the immunological brakes that impede endogenous immune responses against tumours. Antibodies that block the interactions between co-inhibitory receptors and their ligands have delivered very promising clinical responses, as has been shown by recent successful trials targeting the CTLA-4 and PD-1 pathways. In this review, we discuss the mechanisms of action and expression pattern of co-inhibitory receptors on different T cells subsets, emphasising differences between CD4(+) and CD8(+) T cells. We also summarise recent clinical findings utilising immune checkpoint blockade.

Melanoma: oncogenic drivers and the immune system

Advances and in-depth understanding of the biology of melanoma over the past 30 years have contributed to a change in the consideration of melanoma as one of the most therapy-resistant malignancies. The finding that oncogenic BRAF mutations drive tumor growth in up to 50% of melanomas led to a molecular therapy revolution for unresectable and metastatic disease. Moving beyond BRAF, inactivation of immune regulatory checkpoints that limit T cell responses to melanoma has provided targets for cancer immunotherapy. In this review, we discuss the molecular biology of melanoma and we focus on the recent advances of molecularly targeted and immunotherapeutic approaches.

Combined Immune Checkpoint Blockade

There are very few tumor types for which chemotherapy regimens are regularly curative; notable exceptions include testicular cancer and Hodgkin's lymphoma. In both cases combination chemotherapy approaches are required. Thus, it would seem only logical that combination immunotherapy approaches will be required to induce long-term remissions in the majority of cancer patients. Immune checkpoint blockade can be combined with several other interventions, including radiation therapy, chemotherapy and cancer vaccines. However, the observation that T cells that are rendered not-responsive or "exhausted" by recognition of tumor antigens express multiple non-overlapping checkpoint molecules suggests that immunotherapy approaches in which multiple checkpoint molecules are blocked may be particularly active in the clinic.

Present and future of immune checkpoint blockade: Monotherapy to adjuvant approaches

Immune regulation of aggressive tumor growth is often outpaced by tumor up-regulation of ligands that inhibit effector immune responses through the activation of immune checkpoints. A few of such checkpoints include programmed death-1 (PD-1), cytotoxic T lymphocyte associated antigen-4 (CTLA-4), lymphocyte activation gene-3, T-cell immunoglobulin and mucin protein-3, Glucocorticoid-induced TNFR family-related receptor (GITR), and killer cell immunoglobulin like receptor. With the exception of GITR, after binding to their respective ligands these checkpoints induce down-modulation of immune responses to prevent autoimmunity. However, such immune mechanisms are co-opted by tumors to allow rapid tumor cell proliferation. Pre-clinical studies in antibody blockade of PD-1 and CTLA-4 have led to promising augmentation of effector immune responses in murine tumor models, and human antibodies against PD-1 and CTLA-4 alone or in combination have demonstrated tumor regression in clinical trials. The development of immune checkpoint blockade as a potential future immunotherapy has led to increasing interest in combining treatment modalities. Combination checkpoint blockade with chemotherapy and radiation therapy has shown synergistic effects in pre-clinical and clinical studies, and combination checkpoint blockade with bacterial vaccine vectors have produced increased effector immune responses in pre-clinical models. The future of immune checkpoint blockade may be as a powerful adjuvant alongside the current standard of care.

Molecular pathways: coexpression of immune checkpoint molecules: signaling pathways and implications for cancer immunotherapy

The expression of immune checkpoint molecules on T cells represents an important mechanism that the immune system uses to regulate responses to self-proteins. Checkpoint molecules include cytotoxic T lymphocyte antigen-4, programmed death-1, lymphocyte activation gene-3, T-cell immunoglobulin and mucin protein-3, and several others. Previous studies have identified individual roles for each of these molecules, but more recent data show that coexpression of checkpoint molecules occurs frequently on cancer-specific T cells as well as on pathogen-specific T cells in chronic infections. As the signaling pathways associated with each checkpoint molecule have not been fully elucidated, blocking multiple checkpoints with specific monoclonal antibodies results in improved outcomes in several chronic viral infections as well as in a wide array of preclinical models of cancer. Recent clinical data suggest similar effects in patients with metastatic melanoma. These findings support the concept that individual immune checkpoint molecules may function through nonoverlapping molecular mechanisms. Here, we review current data regarding immune checkpoint molecule signaling and coexpression, both in cancer and infectious disease, as well as the results of preclinical and clinical manipulations of checkpoint proteins.

LAG-3 in Cancer Immunotherapy

LAG-3 (CD223) is a cell surface molecule expressed on activated T cells (Huard et al. Immunogenetics 39:213-217, 1994), NK cells (Triebel et al. J Exp Med 171:1393-1405, 1990), B cells (Kisielow et al. Eur J Immunol 35:2081-2088, 2005), and plasmacytoid dendritic cells (Workman et al. J Immunol 182:1885-1891, 2009) that plays an important but incompletely understood role in the function of these lymphocyte subsets. In addition, the interaction between LAG-3 and its major ligand, Class II MHC, is thought to play a role in modulating dendritic cell function (Andreae et al. J Immunol 168:3874-3880, 2002). Recent preclinical studies have documented a role for LAG-3 in CD8 T cell exhaustion (Blackburn et al. Nat Immunol 10:29-37, 2009), and blockade of the LAG-3/Class II interaction using a LAG-3 Ig fusion protein is being evaluated in a number of clinical trials in cancer patients. In this review, we will first discuss the basic structural and functional biology of LAG-3, followed by a review of preclinical and clinical data pertinent to a role for LAG-3 in cancer immunotherapy.

LAG-3 regulates plasmacytoid dendritic cell homeostasis

Lymphocyte activation gene 3 (LAG-3) is a CD4-related, activation-induced cell surface molecule expressed by various lymphoid cell types and binds to MHC class II with high affinity. We have previously shown that LAG-3 negatively regulates the expansion of activated T cells and T cell homeostasis, and is required for maximal regulatory T cell function. In this study, we demonstrate for the first time that LAG-3 is also expressed on CD11c(low)/B220(+)/PDCA-1(+) plasmacytoid dendritic cells (pDCs). Lag3 expression, as determined by real time PCR, was approximately 10-fold greater in pDCs than in either regulatory T cells or activated T effector cells. Activated pDCs also generate approximately 5 times more sLAG-3 than activated T cells. LAG-3-deficient pDCs proliferate and expand more than wild-type pDCs in vivo in response to the TLR9 ligand, CpG. However, the effect of LAG-3 appears to be selective as there was no effect of LAG-3 on the expression of MHC class II, TLR9, and chemokine receptors, or on cytokine production. Lastly, adoptive transfer of either Lag3(+/+) or Lag3(-/-) T cells plus or minus Lag3(+/+) or Lag3(-/-) pDCs defined a role for LAG-3 in controlling pDC homeostasis as well as highlighting the consequences of deregulated Lag3(-/-) pDCs on T cell homeostasis. This raised the possibility of homeostatic reciprocity between T cells and pDCs. Collectively, our data suggests that LAG-3 plays an important but selective cell intrinsic and cell extrinsic role in pDC biology, and may serve as a key functional marker for their study.

Lymphocyte activation gene-3 (CD223) regulates the size of the expanding T cell population following antigen activation in vivo

Lymphocyte activation gene-3 (LAG-3) is a CD4-related, activation-induced cell surface molecule that binds to MHC class II with high affinity. In this study, we used four experimental systems to reevaluate previous suggestions that LAG-3(-/-) mice had no T cell defect. First, LAG-3(-/-) T cells exhibited a delay in cell cycle arrest following in vivo stimulation with the superantigen staphylococcal enterotoxin B resulting in increased T cell expansion and splenomegaly. Second, increased T cell expansion was also observed in adoptive recipients of LAG-3(-/-) OT-II TCR transgenic T cells following in vivo Ag stimulation. Third, infection of LAG-3(-/-) mice with Sendai virus resulted in increased numbers of memory CD4(+) and CD8(+) T cells. Fourth, CD4(+) T cells exhibited a delayed expansion in LAG-3(-/-) mice infected with murine gammaherpesvirus. In summary, these data suggest that LAG-3 negatively regulates T cell expansion and controls the size of the memory T cell pool.

B cell immunodeficiency fails to develop in CD4-deficient mice infected with BM5: murine AIDS as a multistep disease

The immunodeficiency syndrome murine AIDS (MAIDS), caused by the BM5 retrovirus preparation, involves the activation, division, and subsequent anergy of the entire CD4(+) T cell population as well as extensive B cell hyperproliferation and hypergammaglobulinemia, resulting in splenomegaly and lymphadenopathy, followed many weeks later by death. The development of MAIDS requires CD4(+) T cells and MHC class II expression by the infected host, supporting a role for T-B interaction in disease development or progression. To explore this possibility, we examined development of MAIDS in mice deficient in CD4 (CD4 knockout), in which T-B interactions are compromised. We find that in CD4 knockout hosts, BM5 causes T cell immunodeficiency in the remaining T cells but has only a limited ability to induce B cell phenotypic changes, hyperproliferation, hypergammaglobulinemia, or splenomegaly. There is also delayed death of infected mice. This implies that CD4 dependent T-B interaction is needed to induce the B cell aspects of disease and supports a multistep mechanism of disease in which B cell changes follow and are caused by CD4(+) T cell effects.

Interleukin-16 Supports the Migration of Langerhans Cells, Partly in a CD4-Independent Way

Migration of cutaneous dendritic cells is essential for the induction of primary immune responses. Chemotaxis plays an important part in guiding migrating cells through the skin. Therefore, we investigated the influence of interleukin-16, a potent chemoattractant, on the migratory properties of cutaneous dendritic cells. Interleukin-16 added to murine and human skin explant cultures, enhanced emigration of Langerhans cells as well as dermal dendritic cells out of the skin. In contrast to tumor necrosis factor-alpha, intradermally injected interleukin-16 did not reduce the density of Langerhans cells suggesting a chemotactic rather than a mechanistic migration-inducing effect of interleukin-16. In support of these findings, the known migration-promoting effect of tumor necrosis factor-alpha in skin explant cultures could be neutralized by anti-interleukin-16 antibody and vice versa, indicating different but cooperative ways of action for both cytokines. In whole skin explant cultures blocking of the interleukin-16 effect was also achieved with a monoclonal antibody against CD4, the receptor for interleukin-16. In contrast, in cultures of murine epidermis alone no blocking by anti-CD4 became obvious and in CD4-deficient mice Langerhans cell migration in response to interleukin-16 was maintained. This suggests that another receptor for interleukin-16 might be operative for Langerhans cells in the mouse epidermis. Finally, we detected interleukin-16-positive cells in the dermis of skin explants, tumor necrosis factor-alpha-treated and contact allergen-treated skin. Taken together, it seems likely that locally secreted interleukin-16 might serve to enhance the migration of cutaneous dendritic cells and optimize the response to foreign antigen encountering the skin.

Delayed rejection of fetal pig pancreas in CD4 cell deficient mice was correlated with residual helper activity

CD4 cells have been shown to play a dominant role in the rejection of xenografts. Depletion of murine CD4 cells by injecting anti-CD4 antibody prolongs the graft survival, but does not prevent its rejection. For a more stable phenotype, we used genetically modified mice. To test whether the delayed rejection is caused by incomplete depletion of CD4 cells, we evaluated the response to fetal pig pancreas (FPP) xenografts in three types of CD4 cell deficient mice. They are MHC class II deficient mice (MHC II(o/o), CD4 deficient mice (CD4(o/o)) and a novel type of CD4 cell deficient mice (designated GK). GK mice were rendered permanently and completely CD4 deficient by transgenic expression of anti-CD4 antibody, whereas both MHC II(o/o) and CD4(o/o) mice have a residual helper cell population. FPP grafts in wild type mice were rejected within a week, whereas FPP grafts survived up to 4 weeks in MHC II(o/o) and CD4(o/o) mice. Survival of grafts in GK mice was even longer (8 weeks). Differences in histology were also noted. Rejecting grafts in MHC II(o/o) and wild-type mice were infiltrated with both eosinophils and mononuclear cells, whereas the infiltrates in CD4(o/o) and GK mice were exclusively mononuclear cells. Immunohistochemistry showed that they were primarily CD8 cells. The immune response to FPP was clearly different in the three types of CD4 cell deficient mice. Splenocytes of MHC II(o/o) 3 weeks post-transplant with FPP produced substantial amounts of IFN-gamma and IL-5, whereas splenocytes of CD4(o/o) mice produced low levels of IFN-gamma but no detectable IL-5. At similar times, these cytokines were not detected in GK mice. Furthermore, CD4(o/o) mice were capable of mounting helper dependent, although reduced, IgG responses to FPP antigens, while GK mice were not. The above results indicate that residual helper activity in some types of CD4 cell deficient mice could still contribute to xenograft rejection. Caution needs to be exercised where such mice are used as models of CD4 cell deficiency. Also, because there is eventual rejection of xenograft FPP in GK mice which lack detectable helper activity, we argue that these mice are a better model to investigate the involvement of CD4-independent rejection mechanisms.

Both CD4+ and CD8+ T cells are involved in protection against HSV-1 induced corneal scarring

AIM

To determine the relative impact of CD4+ T cells and CD8+ T cells in protecting mice against ocular HSV-1 challenge.

METHODS

CD4+ T cell knockout mice (CD4-/- mice), CD8+ T cell knockout mice (CD8-/- mice), and mice depleted for CD4+ or CD8+ T cells by antibody (CD4+ depleted and CD8+ depleted mice), were examined for their ability to withstand HSV-1 ocular challenge. The parental mice for both knockout mice were C57BL/6J.

RESULTS

These results suggest that: (1) both CD4+ deficient mice (CD4-/- and CD4+ depleted mice) and CD8+ deficient mice (CD8-/-, and CD8+ depleted mice) developed significantly more corneal scarring than their C57BL/6J parental strain; (2) the duration of virus clearance from the eyes of the CD4+ deficient mice was 4 days longer than that of the CD8+ deficient mice; and (3) the severity of corneal scarring in the CD4+ deficient mice was approximately twice that of the CD8+ deficient mice.

CONCLUSIONS

It was reported here that: (1) CD4+ and CD8+ T cells were both involved in protection against lethal ocular HSV-1 infection; and (2) CD4+ and CD8+ T cells were both involved in protection against HSV-1 induced corneal scarring.