Immunization with live, replication-competent Ad-HIV or Ad-SIV recombinant vaccines primes strong antibody responses that develop following administration of booster immunizations with envelope protein. These antibodies display many functional activities. The most desirable for an HIV/AIDS vaccine is broadly neutralizing activity that can prevent infection following exposure to multiple clades or subtypes of the virus that circulate worldwide. However, neither this vaccine approach or others has been able to reliably elicit such antibodies. More readily induced antibodies lack neutralizing activity but broadly react with multiple viral isolates and mediate effector functions via interaction with Fc receptors on a variety of cell types. HIV/SIV infection is initially manifested as small foci of infected cells. Within 2-6 days, virus spreads from these cell foci to draining lymph nodes, leading to systemic infection. The non-neutralizing functional antibody activities can help control the initial viral burden by limiting the spread of virus from the foci of infection. Our replicating Ad-recombinant prime/envelope boost vaccine regimen elicits such activities including antibody dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP) and antibody-dependent complement mediated lysis (ADCML) and has associated them with vaccine-induced protective efficacy in SIV and chimeric SHIV Rhesus macaque models which appropriately mimic HIV infection in people. We have continued to study vaccine-induced antibodies elicited by several vaccine strategies and the role of their non-neutralizing functional activities in protecting against HIV/SIV infection. To further the understanding of mechanisms that lead to enhanced vaccine-induced antibody development we studied T follicular helper (Tfh) cells over the course of vaccination and subsequent SIV exposure in rhesus macaques. Tfh cells are critical for selecting antibody secreting cells with high antigenic affinity in germinal centers (GC) of secondary lymphoid organs. Evaluation of GC TFH and GC B cell dynamics in lymph node biopsies supported a significant role of early GC TFH cells in providing B cell help during initial phases of GC formation. GC TFH responses at day 3 post-mucosal priming were consistent with generation of Env-specific memory B cells in GCs and elicitation of prolonged Env-specific humoral immunity in the rectal mucosa. GC Env-specific memory B cell responses elicited early post-systemic boosting correlated significantly with decreased viremia post-infection. Our results highlighted the importance of early GC TFH cell responses for robust GC maturation and generation of long-lasting SIV specific humoral responses at mucosal and systemic sites. We have also studied B1 cells in the rhesus macaque system. These cells produce natural antibodies, the first line of defense against many pathogens. We found that the frequency of CD11b+ B1 cells in the spleen of macaques chronically infected with SIV correlated positively with viremia and exhausted T cells. The cells were able to induce PD-1 up-regulation on CD4+ T cells, suggesting that these cells may contribute to the regulation of viremia by enhancing T cell exhaustion. Further studies on the mechanisms that regulate B1 cell function may lead to novel strategies for treating HIV/SIV induced disease. Currently we are investigating mucosal-associated invariant T (MAIT) cells over the course of vaccination and infection in our pre-clinical rhesus macaque vaccine trials. MAIT cells contribute to protection against bacterial infections and viral pathogens in a non-antigen-specific manner yet have been shown to be activated and expanded in response to some vaccines. Activated MAIT cells have also been shown to secrete factors that promote B cell differentiation and antibody production, and thus are of particular interest. To date our studies have shown that vaccination with replication-competent adenovirus-SIV recombinants leads to increases in frequency of MAIT cells and their cytokine production both in peripheral blood cells and to a lesser extent in bronchoalveolar lavage cells. We have seen that MAIT cells can activate B cells and stimulate both antibody production as well as generation of tissue-like memory B cells. further studies of these cells should elucidate the role of these cells in vaccine-induced protection, particularly at mucosal sites of infection.
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