The number of virus-specific CD8 T cells increases substantially during an acute infection. Up to 90% of CD8 T cells are virus-specific following lymphocytic choriomeningitis virus (LCMV) infection. In contrast, studies identifying virus-specific CD4 T cell epitopes have indicated that CD4 T cells often recognize a broader array of antigens than CD8 T cells consequently making it difficult to accurately quantify the total magnitude of pathogen-specific CD4 T cell responses. Here we show that following LCMV infection CD4 T cells become CD11ahiCD49d+ and retain this expression pattern into memory. During the effector phase, all of the LCMV-specific IFN-γ+ CD4 T cells display a CD11ahiCD49d+ cell surface expression phenotype. In addition, only memory CD11ahiCD49d+ CD4 T cells make IFN-γ following stimulation. Furthermore, upon secondary LCMV challenge, only CD11ahiCD49d+ memory CD4 T cells from LCMV-immune mice undergo proliferative expansion, demonstrating that CD11ahiCD49d+ CD4 T cells are truly Ag-specific. Using the combination of CD11a and CD49d, we demonstrate that up to 50% of the CD4 T cells are virus-specific during the peak of the LCMV response. Our results indicate that the magnitude of the virus-specific CD4 T cell response is much greater than previously recognized.

SUMMARYThe bacterial pathogen Listeria monocytogenes (Lm) capitalizes on natural killer (NK) cell production of regulatory interleukin (IL)-10 to establish severe systemic infections. Here, we identify regulators of this IL-10 secretion. We show that IL-18 signals to NK cells license their ability to produce IL-10. IL-18 acts independent of IL-12 and STAT4, which co-stimulate IFNγ secretion. Dendritic cell (DC) expression of Nlrp3 is required for IL-18 release in response to the Lm p60 virulence protein. Therefore, mice lacking Nlrp3, Il18, or Il18R fail to accumulate serum IL-10 and are highly resistant to systemic Lm infection. We further show that cells expressing or dependent on Batf3 are required for IL-18-inducing IL-10 production observed in infected mice. These findings explain how Il18 and Batf3 promote susceptibility to bacterial infection and demonstrate the ability of Lm to exploit NLRP3 for the promotion of regulatory NK cell activity.

Respiratory syncytial virus (RSV) is the most common cause of viral lower respiratory tract infections in infants and children under the age of 5. Studies examining RSV infection in susceptible BALB/c mice indicate that both CD4 and CD8 T cells not only contribute to viral clearance but also facilitate RSV-induced disease. However, efforts to understand the mechanisms by which RSV-specific T cells mediate disease following acute RSV infection have been hampered by the lack of defined RSV-specific T cell epitopes. Using an overlapping peptide library spanning each of the RSV-derived proteins, intracellular cytokine staining for gamma interferon was utilized to identify novel RSV-specific CD4 and CD8 T cell epitopes. Five novel CD8 T cell epitopes were revealed within the RSV fusion (F) protein and glycoprotein (G). In addition, five previously unidentified CD4 T cell epitopes were discovered, including epitopes in the phosphoprotein (P), polymerase protein (L), M2-1 protein, and nucleoprotein (N). Though the initial CD4 T cell epitopes were 15 amino acids in length, synthesis of longer peptides increased the frequency of responding CD4 T cells. Our results indicate that CD4 T cell epitopes that are 17 amino acids in length result in more optimal CD4 T cell stimulation than the commonly used 15-mer peptides. IMPORTANCE Respiratory syncytial virus (RSV) is the leading cause of hospitalization for lower respiratory tract infection in children. T cells play a critical role in clearing an acute RSV infection, as well as contributing to RSV-induced disease.Here we examined the breadth of the RSV-specific T cell response, using for the first time an overlapping peptide library spanning the entire viral genome. We identified 5 new CD4 and 5 new CD8 T cell epitopes, including a CD8 T cell epitope within the G protein that was previously believed not to elicit a CD8 T cell response. Importantly, we also demonstrated that the use of longer, 17-mer peptides elicits a higher frequency of responding CD4 T cells than the more commonly used 15-mer peptides. Our results demonstrate the breadth of the CD4 and CD8 T cell response to RSV and demonstrate the importance of using longer peptides when stimulating CD4 T cell responses. R espiratory syncytial virus (RSV), a single-stranded negativesense RNA paramyxovirus, is the leading cause of hospitalizations for lower respiratory tract infections in young children (1). RSV is a ubiquitous pathogen, infecting 30 to 60% of children during their first year of life and up to 90% of children by their second year of life (2-4). Furthermore, approximately 3% of RSVinfected children require hospitalization (5). In total, it is estimated that worldwide, RSV is responsible for 3.4 million acute lower respiratory tract infections annually in children under the age of 5, resulting in up to 196,000 yearly fatalities (6). In addition to children, the elderly are also susceptible to severe RSV-induced disease (7).BALB/c mice are susceptible to RSV infection and have been used extensively to ...

Lymphocytic choriomeningitis virus (LCMV) causes a systemic infection in mice with virus replication occurring in both peripheral tissues and secondary lymphoid organs. Because of the rapid systemic dissemination of the virus, the secondary lymphoid organs responsible for the induction of the LCMV-specific CD8 T cell response are poorly defined. We show that the mediastinal lymph node (MedLN) serves as the primary draining lymph node following LCMV infection. In addition, we demonstrate that the MedLN is responsible for priming the majority of the virus-specific CD8 T cell response. Following resolution of the acute infection, the draining MedLN exhibits characteristics of a reactive lymph node including an increased presence of germinal center B cells and increased cellularity for up to 60 days post-infection. Furthermore, the reactive MedLN harbors an increased frequency of CD62L− effector memory CD8 T cells as compared to the non-draining lymph nodes. The accumulation of LCMV-specific CD62L− memory CD8 T cells in the MedLN is independent of residual antigen and is not a unique feature of the MedLN as footpad infection with LCMV leads to a similar increase of virus-specific CD62L− effector memory CD8 T cells in the draining popliteal lymph node. Our results indicate that CD62L− effector memory CD8 T cells are granted preferential access into the draining lymph nodes for an extended time following resolution of an infection.

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract disease in young children. Premature infants, immunocompromised individuals and the elderly exhibit the highest risk for the development of severe RSV-induced disease. Murine studies demonstrate that CD8 T cells mediate RSV clearance from the lungs. Murine studies also indicate that the host immune response contributes to RSV-induced morbidity as T-cell depletion prevents the development of disease despite sustained viral replication. Dendritic cells (DCs) play a central role in the induction of the RSV-specific adaptive immune response. Following RSV infection, lung-resident DCs acquire viral antigens, migrate to the lung-draining lymph nodes and initiate the T-cell response. This article focuses on data generated from both in vitro DC infection studies and RSV mouse models that together have advanced our understanding of how RSV infection modulates DC function and the subsequent impact on the adaptive immune response.

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CD4 T cells play a vital role in orchestrating adaptive immune responses. Measuring the magnitude of the CD4 T cell response to a given pathogen is currently hindered by the lack of complete knowledge of all of the CD4 T cell epitopes. In cases where most or all of the CD4 T cell epitopes have been mapped, current estimates based on intracellular cytokine production for cytokines such as IFN-γ may also under-represent the frequency of antigen-specific CD4 T cells because of the high heterogeneity in cytokine production exhibited by differentiated effector CD4 T cells. Here we show that following lymphocytic choriomeningitis virus (LCMV) infection, T cell receptor-transgenic CD4 T cells specific to LCMV upregulate CD11a and CD49d and that expression of these adhesion molecules remains stable into memory. Expression of these integrins requires cognate antigen and is not modulated non-specifically by inflammatory signals such as stimulation with LPS or CPG. Additionally, CD11a and CD49d expression is upregulated on endogenous CD4 T cells following acute LCMV infection as well as all LCMV-specific CD4 T cells that respond to previously identified CD4 T cell epitopes. Using these markers we estimate the total magnitude of the virus-specific CD4 T cell response to multiple pathogens and our results indicate that the CD4 T cell response is substantially larger than previously thought.

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