Rotations: Projects in the following areas

CD4 T cell memory. We can generate in vitro CD4 T cells that are for all extents and purposes identical to the memory CD4 T cells observed in vivo. In situ memory CD4 T cells persist at very low frequency and in most instances it is difficult to analyze them thoroughly using tetramer or multimer reagents both because of their low frequency and the relatively low specific affinity of the Class II reagents for the TcR. Being able to generate “recent” memory cells in vitro gives us the opportunity to analyze in detail the mechanisms by which memory CD4 T cells may contribute to secondary immunity and immune protection. Once identified. will determine if particular pathways are relevant in situ. We have discovered that memory CD4 T cells are superior in protection to those from the primary response and that they display pleitropic functions that have not been previously identified.

Cytotoxic CD4 Effectors. One unexpected function of CD4 T cells we identified is robust, perforin-dependent cytotoxic activity (7, 16). Cytotoxic activity by CD4 T cells has been reported in response to several infections, but its significance has not heretofore been established. We find that “Th1 polarized” CD4 effectors and more relevantly the CD4 effectors found in the lung following influenza infection, are able to kill infected or peptide-pulsed Class II-bearing targets and we find that this perforin dependent activity can work together with antibody to provide protection against lethal influenza challenge in otherwise unprimed mice (17). We also will determine when memory CD4 T cells re-express cytotoxic activity following in vivo challenge and how cytotoxic killing contributes to the memory cell mediated protection. We want to analyze how the program of cytotoxicity is induced in these “ThCTL”.

Memory CD4 Cells Enhance an Innate Response. Memory cells can act by becoming secondary effectors but they are best characterized by their ability respond rapidly even to low antigen doses, by producing cytokines and chemokines and it is assumed that these initial abilities are key to memory cell activity. However this assumption has not been directly demonstrated and the mechanisms involved and how they might contribute to immunity have not been elucidated. We find that restimulation of memory CD4 T cells leads to a markedly enhanced early innate response to influenza infection both in the lung and systemically (17). This response peaks 2-3 days after infection and is transient. We see enhanced production of IL-6 locally and systemically and enhanced production a broad spectrum of innate inflammatory mediators in the lung. We hope to determine in detail the pathways leading to optimum memory T cell-induced innate responses. We suggest a concurrent activation of CD4 memory cells may be an effective adjuvant for vaccination and will be testing this hypothesis as well as mechanisms that lead to viral control over the next couple of years.

Aging and Immunity: The study of the impact of aging on CD4 T cell function at the naïve and memory stages has lead us to better understanding of fundamental aspects of T cell behavior in addition to providing important information about what vaccine approaches are most likely to be successful for the aged.

Nature of CD4 Aging “Defects”. We have identified multiple levels at which aged naïve CD4 T cells are defective including initial early response to TcR triggering, IL-2 production, expansion and effector generation. The defects in helper function and in the 2⁰ response of memory cells generated from aged naïve CD4 T cells are particularly dramatic. In recent studies we have recapitulated memory age-associated defects in recently generated memory cells in vitro (23). In vitro aged memory cells are markedly defective in production of particular cytokines, IL-4, IL-5 and IL-2, while others are little affected (IFNg, IL-10, TNF), suggesting the aging “defect” may be restricted to loss of potential to make certain CD4 T cell subsets, such as those responsible for help to B cells .We will pursue this hypothesis both in vitro and in vivo.

We have found that aged naïve CD4 T cells are in fact longer-lived than their naïve counterparts (23). We find that reduced expression of Bim, a proapoptotic protein, is responsible for this increase in lifespan and that the increase in lifespan is required for the development of aging defects. We are now investigating the factors that regulate Bim expression.

Enhancement by Inflammatory Cytokines. We have shown that the defective effector generation responses of naïve CD4 T cells from aged animals can be overcome by IL-2 and by proinflammatory cytokines, in particular a mix of IL-1, IL-6 and TNF (20,21). We want to determine the extent to which the inflammatory cytokines can reverse the heritable aging defects and restore functional memory generation. We have developed in vitro models to test the generation of effectors and memory. In these models we are evaluating TLR agonists for their ability to restore aged naïve CD4 responses to peptide-pulsed derived dendritic cells (DC). Pre-incubation of DC with many TLR agonists, enhances expansion of the responding aged naïve CD4 T cells via a mechanism dependent on IL-6 that acts in part by increasing survival of the developing effector population. We plan to use this model to identify the extent of the rescue and what pathways and mechanisms contribute with the ultimate goal of evaluating whether specific TLR stimulation or stimulation of specific pathways might improve the response of the elderly to vaccines. We want to see if we can reproduce this model with human naïve CD4 T cells and DC derived from peripheral blood and if TLR agonists can also enhance those responses.