Quantitative analysis of antigen-mediated CD4 T cell - CD4 T cell cooperation determining the Th1/Th2 phenotype of a primary immune response

Several variables have been found to affect the Th1/Th2 differentiation of newly activated CD4 T cells. This phenotype can be critical in determining effectiveness of immune responses. Experiments in this thesis were undertaken to better define the in-vivo cellular interactions involved in determining the Th1/Th2 phenotype of newly activated CD4 T cells.Lethally irradiated BALB/c mice reconstituted with a constant number of syngeneic, naive spleen cells were challenged with xenogeneic red blood cells (XRBC) conjugated to ovalbumin (OVA) and the Th1/Th2 phenotype of the anti-XRBC response assessed. Antigen-specific interferon-gamma (IFN-g) and interleukin-4 (IL-4) secreting cells obtained from spleens of immunized mice were enumerated by an ELISPOT assay; the relative number of IFN-g- and IL-4-producing cells is taken as a relative measure of Th1 and Th2 components of the response. When challenged with a ‘standard’ dose of XRBC-OVA, predominant Th1 responses are generated; when challenged with a ten-fold lower dose, such reconstituted mice do not generate significant responses. This adoptive transfer system was employed to explore further the relationships between quantitative changes in the dose of immunizing antigen and the number of responding antigen-specific CD4 T cells, and the Th1/Th2 phenotype of immune responses generated. Unprimed transgenic CD4 T cells specific for OVA can modulate the Th1/Th2 phenotype of the anti-XRBC response upon immunization with XRBC-OVA. Addition of a small number of naive transgenic spleen cells to the standard reconstituting population of normal spleen cells results in the generation of significant numbers of SRBC-specific Th2 cells when mice are challenged with a ‘standard’ dose, or can generate predominant Th1 responses when mice are challenged with a ten-fold lower dose. Transgenic cells only impact the Th1/Th2 phenotype of CD4 T cells specific for XRBC when OVA is linked to the XRBC. That CD4 T cells specific for different antigens cooperate only through the recognition of linked antigenic determinants has important implications for many aspects of immune regulation. Observations further show that thymocytes from transgenic mice can influence the XRBC-specific response phenotype in an identical manner as transgenic spleen cells, suggesting that previously polarized pro-Th1/Th2 cells are not required in the cooperative events influencing Th1/Th2 phenotype of newly activated CD4 T cells.These observations lead to a quantitative description, whereby antigen-mediated CD4 T cell cooperation can affect the Th1/Th2 phenotype of a primary antigen-specific immune response, and provide a context for further analysis at the molecular level

Early Programming And Late-Acting Checkpoints Governing The Development Of Cd4 T-Cell Memory

CD4 T cells contribute to protection against pathogens through numerous mechanisms. Incorporating the goal of memory CD4 T-cell generation into vaccine strategies therefore offers a powerful approach to improve their efficacy, especially in situations where humoral responses alone cannot confer long-term immunity. These threats include viruses such as influenza that mutate coat proteins to avoid neutralizing antibodies, but that are targeted by T cells that recognize more conserved protein epitopes shared by different strains. A major barrier in the design of such vaccines is that the mechanisms controlling the efficiency with which memory cells form remain incompletely understood. Here, we discuss recent insights into fate decisions controlling memory generation. We focus on the importance of three general cues: interleukin-2, antigen and co-stimulatory interactions. It is increasingly clear that these signals have a powerful influence on the capacity of CD4 T cells to form memory during two distinct phases of the immune response. First, through ‘programming’ that occurs during initial priming, and second, through ‘checkpoints’ that operate later during the effector stage. These findings indicate that novel vaccine strategies must seek to optimize cognate interactions, during which interleukin-2-, antigen- and co-stimulation-dependent signals are tightly linked, well beyond initial antigen encounter to induce robust memory CD4 T cells

Mouse Models Reveal Role of T-Cytotoxic and T-Reg Cells in Immune Response to Influenza: Implications for Vaccine Design

Immunopathologic examination of the lungs of mouse models of experimental influenza virus infection provides new insights into the immune response in this disease. First, there is rapidly developing perivascular and peribronchial infiltration of the lung with T-cells. This is followed by invasion of T-cells into the bronchiolar epithelium, and separation of epithelial cells from each other and from the basement membrane leading to defoliation of the bronchial epithelium. The intraepithelial reaction may involve either CD8 or CD4 T-cytotoxic cells and is analogous to a viral exanthema of the skin, such as measles and smallpox, which occur when the immune response against these infections is activated and the infected cells are attacked by T-cytotoxic cells. Then there is formation of B-cell follicles adjacent to bronchi, i.e., induced bronchial associated lymphoid tissue (iBALT). iBALT reacts like the cortex of a lymph node and is a site for a local immune response not only to the original viral infection, but also related viral infections (heterologous immunity). Proliferation of Type II pneumocytes and/or terminal bronchial epithelial cells may extend into the adjacent lung leading to large zones filled with tumor-like epithelial cells. The effective killing of influenza virus infected epithelial cells by T-cytotoxic cells and induction of iBALT suggests that adding the induction of these components might greatly increase the efficacy of influenza vaccination

New Insights Into The Generation Of CD4 Memory May Shape Future Vaccine Strategies For Influenza

Influenza viral evolution presents a formidable challenge to vaccination due to the virus’ ability to rapidly mutate to evade immune responses. Live influenza infections generate large and diverse CD4 effector T cell responses that yield highly protective, long-lasting CD4 T cell memory that can target conserved viral epitopes. We review advances in our understanding of mechanisms involved in generating CD4 T cell responses against the influenza A virus (IAV), focusing on specialized follicular helper (TFH) and CD4 cytotoxic (ThCTL) effector subsets and on CD4 T cell memory. We also discuss two recent findings in context of enhancing vaccine responses. First, that helper T cells require priming with APC secreting high levels of IL-6. Second, that the transition of IAV-generated effectors to memory depends on IL-2, co-stimulation and antigen signals, just before effectors reach peak numbers, defined as the memory checkpoint. The need for these signals during the checkpoint could explain why many current influenza vaccines are poorly effective and elicit poor cellular immunity. We suggest that CD4 memory generation can be enhanced by re-vaccinating at this time. Our best hope lies in a universal vaccine that will not need to be formulated yearly against seasonal antigenically-novel influenza strains and will also be protective against a pandemic strain. We suggest a vaccine approach that elicits a powerful T cell response, by initially inducing high levels of APC activation and later providing antigen at the memory checkpoint, may take us a step closer to such a universal influenza vaccine

New Insights Into The Generation Of Cd4 Memory May Shape Future Vaccine Strategies For Influenza

Influenza viral evolution presents a formidable challenge to vaccination due to the virus\u27 ability to rapidly mutate to evade immune responses. Live influenza infections generate large and diverse CD4 effector T cell responses that yield highly protective, long-lasting CD4 T cell memory that can target conserved viral epitopes. We review advances in our understanding of mechanisms involved in generating CD4 T cell responses against the influenza A virus (IAV), focusing on specialized follicular helper (TFH) and CD4 cytotoxic (ThCTL) effector subsets and on CD4 T cell memory. We also discuss two recent findings in context of enhancing vaccine responses. First, helper T cells require priming with APC secreting high levels of IL-6. Second, the transition of IAV-generated effectors to memory depends on IL-2, costimulation and antigen signals, just before effectors reach peak numbers, defined as the memory checkpoint. The need for these signals during the checkpoint could explain why many current influenza vaccines are poorly effective and elicit poor cellular immunity. We suggest that CD4 memory generation can be enhanced by re-vaccinating at this time. Our best hope lies in a universal vaccine that will not need to be formulated yearly against seasonal antigenically novel influenza strains and will also be protective against a pandemic strain. We suggest a vaccine approach that elicits a powerful T cell response, by initially inducing high levels of APC activation and later providing antigen at the memory checkpoint, may take us a step closer to such a universal influenza vaccine