Immune Correlates of Cross-Reactive Neutralizing Antibodies in HIV-1 Infection

Abstract

In spite of more than three decades of rigorous investigation, there remains no licensed vaccine against HIV-1. A major challenge in developing an effective vaccine is the extensive diversity in HIV-1, varying by as much as 35% amino acid sequence in the HIV-1 envelope protein (Env), which is the sole target of neutralizing antibodies. Therefore, an effective HIV-1 vaccine will likely require elicitation of potent neutralizing antibodies able to neutralize diverse heterologous viral isolates. Such cross-reactive neutralizing antibodies (CRNA) develop in a minority of HIV-1 infected subjects, with broad and potent monoclonal antibodies having been isolated from some. In this thesis, I have concentrated on identifying immunological correlates associated with the development of these responses during HIV-1 infection and the potential mechanisms involved. I examined immunological factors in a cohort of HIV-1 infected subjects who have been monitored regularly during their infection; starting soon after infection and followed for up to several years and for whom the development of serum CRNA was previously documented in detail (Mikell et al., 2011b). In my first aim, I evaluated the phenotypes and frequencies of peripheral T follicular helper-like (pTFH) cells and Env-specific B cells, plasma cytokines, and B cell transcriptional profiles during early infection and at ~2.5 years post-infection when CRNA becomes detectable in some subjects. Consequently, I demonstrated significant correlations between the frequency of pTFH cells, plasma levels of CXCL13, and development of CRNA, independent of plasma viral load. Early in infection the subjects who later developed CRNA had a higher frequency of pTFH cells, reflecting levels found in HIV-1 uninfected subjects, suggesting that there may be underlying mechanisms critical for maintaining pTFH cells in those individuals in early infection. In addition, we show that B cells from these subjects expressed more activation-induced cytidine deaminase (AID) in the first year post-infection, and that AID transcript levels correlated with the frequency of pTFH cells. The pTFH cells from these individuals were also more effective at inducing class-switching in autologous B cells <italic>in vitro<italic>. Therefore, our results directly link the development of CRNA against HIV-1 with pTFH cell frequency and function. Furthermore, as elevated levels of the chemokine, CXCL13, were observed in the plasma of subjects with CRNA, in my second aim, I sought to determine potential pathways which may induce CXCL13 during HIV-1 infection. To this end, I identified two potential mechanisms of HIV-1 induced CXCL13 secretion; one due to direct TLR7/8 activation in monocytes by ssHIV-1 RNA and the second due to TLR7 induction of type I interferon (IFN) by pDCs and subsequent IFN stimulation of monocytes. Taken together, there may be differences in aspects of the innate immune responses during acute/early HIV-1 infection that predispose or contribute to some individuals developing CRNA later in infection. Furthermore these studies identified pathways potentially contributing to the development of CRNA as well as a possibly predictive `signature' of CRNA development following HIV-1 infection and vaccination.