CD4 T cell cooperation is required for the in vivo activation of CD4 T cells

Poly(I:C) and R848 ligands show better adjuvanticity to induce B and T cell responses against the antigen(s)

Poly(I:C) and R848, synthetic ligands that activate Toll-like receptor 3 (TLR3) and TLR7/8 respectively, have been well-established for their ability to stimulate the immune system and induce antigen-specific immune responses. These ligands are capable of inducing the production of cytokines and chemokines, and hence support the activation and differentiation of B and T cells. We saw the long-lasting and perdurable immune responses by these adjuvants essentially required for an efficacious subunit vaccine. In this study, we investigated the potential of poly(I:C) and R848 to elicit B and T cell responses to the OVA antigen. We assessed the stimulatory effects of these ligands on the immune system, their impact on B and T cell activation, and their ability to enhanced generation of B and T cells. Collectively, our findings contribute to the understanding how poly(I:C) and R848 can be utilized as an adjuvant system to enhance immune responses to protein-based subunit vaccines. In the end, this work provides insights for the development of novel vaccination strategies and improving the vaccine efficacy. Present work shall help formulate newer strategies for subunit vaccines to address the infectious diseases.

Recent Advances in Immune-Based Therapies for Type 1 Diabetes

BACKGROUND

Type 1 diabetes (T1D) is a chronic autoimmune disease characterized by progressive destruction of the pancreatic beta cells, leading to a lifelong dependence on insulin. It is associated with an increased morbidity and mortality from diabetes-related complications and a significant treatment burden. However, there has been substantial progress in therapeutic strategies that can affect the course of the disease.

SUMMARY

This review addresses advances in immunotherapy aimed at preserving residual beta-cell function in individuals with a recent onset of T1D and arresting the disease in pre-symptomatic stages. Recent and ongoing clinical trials have investigated the efficacy and safety of various immunotherapeutic strategies aimed at targeting several mechanisms of autoimmunity, which are thought to be important in disease pathogenesis, and therapies that also address beta-cell health. So far, T-cell-directed therapies that led to a favourable balance between T-effector cell depletion or modulation and preservation or expansion of regulatory T cells have shown the most success. Furthermore, regarding the timing of intervention, teplizumab was the first immunomodulatory agent to demonstrate a significant delay in disease progression in high-risk individuals before clinical onset.

KEY MESSAGES

As more targeted immune interventions with potentially fewer side effects are closer to the translation into clinical practice, some new challenges may need to be addressed. The use of combination approaches that include immunotherapeutic strategies targeting different aspects of the immune system and interventions that improve beta-cell health may be required, along with the use of individualized patient-tailored approaches, a move towards early intervention, and a focus on patient-reported outcome measures.

Pathogen Dose in Animal Models of Hemorrhagic Fever Virus Infections and the Potential Impact on Studies of the Immune Response

Viral hemorrhagic fever viruses come from a wide range of virus families and are a significant cause of morbidity and mortality worldwide each year. Animal models of infection with a number of these viruses have contributed to our knowledge of their pathogenesis and have been crucial for the development of therapeutics and vaccines that have been approved for human use. Most of these models use artificially high doses of virus, ensuring lethality in pre-clinical drug development studies. However, this can have a significant effect on the immune response generated. Here I discuss how the dose of antigen or pathogen is a critical determinant of immune responses and suggest that the current study of viruses in animal models should take this into account when developing and studying animal models of disease. This can have implications for determination of immune correlates of protection against disease as well as informing relevant vaccination and therapeutic strategies.

On Analyzing How the Th1/Th2 Phenotype of an Immune Response Is Determined: Classical Observations Must Not Be Ignored

How an antigen interacts differently with lymphocytes and other cells of the immune system, to result in the generation of distinct classes of immunity, is one of the most basic questions of immune regulation. Understanding the nature of these "decision criteria" is central to developing effective medical interventions. Clinical observations lead to the recognition that much disease is due to an inappropriate class of immunity being generated, inappropriate because damaging, as in autoimmunity and allergies, or inappropriate because ineffective, against pathogens and cancer. I argue that the prevalent, contemporary conceptual frameworks, employed to analyze the nature of the decision criterion controlling the Th1/Th2 phenotype of the immune response, are implausible, as they ignore pertinent, classical observations. I outline reasons for favoring a different framework, that takes these observations into account, and explore its pertinence to the design of strategies of medical intervention.

The history of the two-signal model of lymphocyte activation: A personal perspective

The first ideas leading to The Two-Signal Model of lymphocyte activation were published 50 years ago, but the model was not realized in one sitting. I describe the three phases that led to its contemporary formulations. A motivation underlying all these models was to generate a minimal description of what is required for antigen to inactivate and activate mature lymphocytes that, at the same time, accounts for how peripheral tolerance is achieved. I suggest the two signal model has not only provided a substantiated framework for understanding how antigen interacts differently with B cells and CD8 T cells, to result in their inactivation and activation, but its postulates are pertinent to contemporary issues concerning the inactivation and activation of CD4 T cells.

A Conversation with Cohn on the Activation of CD4 T Cells

Despite an agreement on most issues surrounding models for how lymphocytes are activated and inactivated, and arising out of the 1970 Two Signal Model of lymphocyte activation, Cohn and I have different perspectives on two critical issues concerning the activation of CD4 T cells. One issue is the origin of the first effector T helper (eTh) cells, postulated by both of us to be required to optimally activate precursor Th (pTh), that is naïve CD4 T cells, to further generate eTh cells. The second issue arises from our agreement that the antigen‐dependent CD4 T cell cooperation, that we both postulate is required to activate naïve CD4 T (pTh) cells, most likely is mediated by the operational recognition of linked epitopes. Although agreeing on the centrality of this operational mechanism, we disagree about how it might be realized at the molecular/cellular level. I respond here to issues raised by Cohn concerning these two mechanistic questions, in his response to my recent article on the activation and inactivation of mature CD4 T cells.

The activation and inactivation of mature CD4 T cells: a case for peripheral self-nonself discrimination

The establishment of central tolerance to most self-antigens results in a repertoire of mature peripheral lymphocytes specific for foreign and peripheral self-antigens. The framework that single, mature lymphocytes are inactivated by antigen, whereas their activation requires lymphocyte cooperation, accounts for diverse observations and incorporates a mechanism of peripheral tolerance. This framework accounts for the generalizations that the sustained activation by antigen of most B cells and CD8 T cells requires CD4 T helper cells; in the absence of CD4 T cells, antigen can inactivate these B cells and CD8 T cells. In this sense, CD4 T cells are the guardians of the fate of most B cells and CD8 T cells when they encounter antigen. I argue here that the single-lymphocyte/multiple-lymphocyte framework for the inactivation/activation of lymphocytes also applies to CD4 T cells. I consider within this framework a model for the activation/inactivation of CD4 T cells that is consistent with the large majority of contemporary observations, including significant clinical observations. I outline the grounds why I feel this model is more plausible than the contemporary and predominant pathogen-associated molecular pattern (PAMP) and Danger Models for CD4 T cell activation. These models are based upon what I consider the radical premise that self–nonself discrimination does not exist at the level of mature CD4 T cells. I explain why I feel this feature renders the PAMP and Danger Models somewhat implausible. The model I propose, in contrast, is conservative in that it embodies such a process of self–nonself discrimination.

The number of responding CD4 T cells and the dose of antigen conjointly determine the TH1/TH2 phenotype by modulating B7/CD28 interactions

Our previous in vivo studies show that both the amount of Ag and the number of available naive CD4 T cells affect the Th1/Th2 phenotype of the effector CD4 T cells generated. We examined how the number of OVA-specific CD4 TCR transgenic T cells affects the Th1/Th2 phenotype of anti-SRBC CD4 T cells generated in vivo upon immunization with different amounts of OVA-SRBC. Our observations show that a greater number of Ag-dependent CD4 T cell interactions are required to generate Th2 than Th1 cells. We established an in vitro system that recapitulates our main in vivo findings to more readily analyze the underlying mechanism. The in vitro generation of Th2 cells depends, as in vivo, upon both the number of responding CD4 T cells and the amount of Ag. We demonstrate, using agonostic/antagonistic Abs to various costimulatory molecules or their receptors, that the greater number of CD4 T cell interactions, required to generate Th2 over Th1 cells, does not involve CD40, OX40, or ICOS costimulation, but does involve B7/CD28 interactions. A comparison of the level of expression of B7 molecules by APC and CD4 T cells, under different conditions resulting in the substantial generation of Th1 and Th2 cells, leads us to propose that the critical CD28/B7 interactions, required to generate Th2 cells, may directly occur between CD4 T cells engaged with the same B cell acting as an APC.

Antigen presenting B cells facilitate CD4 T cell cooperation resulting in enhanced generation of effector and memory CD4 T cells

We show that the in vivo generation of cytokine-producing CD4 T cells specific for a given major histocompatibility class-II (MHCII)-binding peptide of hen egg lysozyme (HEL) is facilitated when mice are immunized with splenic antigen presenting cells (APC) pulsed with this HEL peptide and another peptide that binds a different MHCII molecule. This enhanced generation of peptide-specific effector CD4 T cells requires that the same splenic APC be pulsed with both peptides. Pulsed B cells, but not pulsed dendritic cells (DCs), can mediate CD4 T cell cooperation, which can be blocked by disrupting OX40-OX40L (CD134-CD252) interactions. In addition, the generation of HEL peptide-specific CD4 T cell memory is greater when mice are primed with B cells pulsed with the two peptides than with B cells pulsed with the HEL- peptide alone. Based on our findings, we suggest CD4 T cell cooperation is important for vaccine design, underlies the phenomenon of "epitope-spreading" seen in autoimmunity, and that the efficacy of B cell-depletion in the treatment of human cell-mediated autoimmune disease is due to the abrogation of the interactions between autoimmune CD4 T cells that facilitates their activation.

Direct demonstration of CD4 T cell cooperation in the primary in vivo generation of CD4 effector T cells

Many observations bear upon the cellular and molecular requirements for CD4 T cell activation. The interaction of CD4 T cells with dendritic cells (DC), central to the induction of most immune responses, is the most studied. However, leukocytes other than DC can dramatically affect the induction and differentiation of CD4 T cells into effector cells. We recently provided indirect evidence that in vivo CD4 T cooperation facilitates the activation of CD4 T cells. Here, we demonstrate that the activation of CD4 T cells, specific for the hen egg lysozyme (HEL)(105) (-120) peptide, is optimally achieved when BALB/c mice are immunized with additional MHC class II-binding HEL peptides in incomplete Freund's adjuvant. This cooperation cannot be mimicked by the coadministration of LPS or of an agonistic antibody to CD40, at the time of immunization. In contrast, OX40-OX40L interactions are necessary for CD4 T cell cooperation in that an OX40 agonistic antibody can replace, and an OX40L-blocking antibody can abrogate, CD4 T cell cooperation in situations where such cooperation would otherwise enhance the activation of CD4 T cells.