Protective immunity, whether induced by infection or by vaccination, relies on the generation of memory B & T cells in sufficient quantity, and of sufficient quality. The success of this process is heavily dependent on help that is provided by CD4+ T cells. CD4+ T cells play a central role in orchestrating the adaptive the immune response but the mechanism by which their abundance is regulated, the importance of recurrent antigen contact, and the means by which they provide help to CD8+ T cells all remain uncertain. The overall goals of this application are (i) to continue our studies of how CD4+ T cell quantity and quality are regulated in vivo by IFN, and how they are affected by in vivo contact with persistent virus antigen (Aims 1 & 2); and (ii) to investigate how CD4+ T cells provide the help that is required for the development of good antiviral CD8+ T cell memory (Aims 3 & 4). Aim 1. To determine how direct IFN signaling exerts its profound effects on the abundance of primary and memory CD4+ T cells. We have shown that CD4+ T cells that can respond directly to IFN are 100- fold more abundant in the memory pool. We will determine when the effect of IFN is exerted; what changes in gene expression occur as a result; and which cells produce this important cytokine. Aim 2. To evaluate the effects of prolonged MHC class II antigen presentation on CD4+ T cell quantity and quality. It is well-established that, during chronic infection, persistent antigen can dramatically alter the T cell response. However, the issue of antigen persistence is much less well-defined in models of acute viral infection. We have found that MHC class II presents virus antigen for weeks after acute virus infection. We shall measure the effects of persistent antigen on CD4+ T cell functions. Does antigen- driven IFN production sculpt the CD4+ T cell response? Aim 3. To investigate the help that is provided by virus-specific and non-specific CD4+ T cells. My lab was among the first to show that CD4+ T cells played a key role in establishing CD8+ T cell memory following acute virus infection, an observation that has been confirmed and extended by a number of labs. Surprisingly, CD4+ T cells seem to be able to provide help to CD8+ T cells in a non-specific (noncognate) fashion. We shall compare the helpfulness of CD4+ T cells that are specific for a viral epitope, with the helpfulness of CD4+ T cells that cannot recognize the virus. Aim 4. To determine if IFN is a key mediator of CD4+ T cell help. The final Aim draws together two threads: (i) the fact that CD4+ T cell help is required for the development of good CD8+ T cell memory and (ii) our recent finding that IFN is vitally important for the development of CD8+ T cell memory. Might IFN be the link by which CD4+ T cells help the development of CD8+ T cell memory? Is the role of IFN the same for specific & non-specific CD4+ helper T cells?