HIV-1 vaccine development : evaluating mechanisms of CD8+ T cell avidity

Abstract

CD8+ T cells play pivotal roles in anti-viral immunity. In particular, CD8+ T cells that make high avidity interactions with virus-infected cells are extremely efficient in controlling virus infections including human immunodeficiency virus (HIV)-1. Therefore, induction of high avidity anti-viral CD8+ T cells following vaccination is expected to be beneficial for conferring protection against HIV-1. Our laboratory has shown that mucosal HIV-1 recombinant pox viral prime-boost vaccination can induce HIV-specific CD8+ T cells with reduced interleukin (IL)-4 and IL-13 cytokine expression and higher avidity compared to systemic vaccine delivery. To understand how these cytokines regulated CD8+ T cell avidity, the PhD studies initially evaluated the expression of receptors for these cytokines on immune cells using flow cytometry following a range of viral infections (e.g. pox viruses and influenza virus) in mice. Results indicated that unlike other IL-4/IL-13 receptor subunits, IL-4 receptor alpha (IL-4Ra) was significantly down-regulated on anti-viral CD8+ T cells in a T cell receptor (TCR) and cognate antigen dependent manner. These studies also showed that up-regulation of IL-4Ra on naive CD8+ T cells most likely resulting from signal transducer and activator of transcription 6 (STAT6) signaling correlated with poor avidity anti-viral CD8+ T cell cytokine responses. Poor avidity anti-viral CD8+ T cells that developed in this instance as well as following systemic HIV-1 recombinant pox viral prime-boost vaccination expressed lower CD8 co-receptor densities. Interestingly, mucosal and IL-13 inhibitor HIV-1 recombinant pox viral prime-boost vaccination strategies prevented significant down-regulation of CD8 densities and enhanced avidity of anti-viral CD8+ T cells. Collectively, data suggest that enhancing responsiveness of naive CD8+ T cells to IL-4 and IL-13 (i.e. up-regulation of IL-4Ra) primes poor avidity anti-viral CD8+ T cells with reduced CD8 densities during virus infection and recombinant HIV-1 pox viral prime-boost vaccination. Given the poor capacity of current assays to evaluate CD8+ T cell avidity in vivo following HIV-1 vaccination, the current PhD studies also evaluated the use of a fluorescent target array (FTA) for this purpose. The FTA assay involves measuring in vivo T cell responses against peptide-pulsed splenocytes (targets) that have unique fluorescent signatures (e.g. over 200 signatures) after injection into vaccinated mice. This assay was utilized to effectively screen for 24 HIV-1 recombinant pox viral vaccination regimes for the capacity to induce high avidity and epitope variant cross-reactive CD8+ T cells as well as T helper responses in vivo. Overall, the FTA was found to be an extremely versatile assay for screening vaccines that could induce high quality T cell responses in vivo using pre-clinical models. Currently, the lack of knowledge regarding mechanisms that affect CD8+ T cell avidity and methods that evaluate CD8+ T cell avidity in vivo is a major barrier for developing efficacious HIV-1 vaccines. Therefore, the findings from the PhD research studies regarding how IL-4 and IL-13 regulate avidity and the use of a FTA to measure avidity in vivo could be exploited to foster future development of more efficacious HIV-1 vaccines.