Effective strategies are needed to block mucosal transmission of human immunodeficiency virus type 1 (HIV-1). Here, we address a crucial question in HIV-1 pathogenesis: whether infected donor mononuclear cells or cell-free virus plays the more important role in initiating mucosal infection by HIV-1. This distinction is critical, as effective strategies for blocking cell-free and cell-associated virus transmission may be different. We describe a novel ex vivo model system that utilizes sealed human colonic mucosa explants and demonstrate in both the ex vivo model and in vivo using the rectal challenge model in rhesus monkeys that HIV-1-infected lymphocytes can transmit infection across the mucosa more efficiently than cell-free virus. These findings may have significant implications for our understanding of the pathogenesis of mucosal transmission of HIV-1 and for the development of strategies to prevent HIV-1 transmission. N ovel microbicide and vaccine candidates for human immunodeficiency virus type 1 (HIV-1) are being evaluated preclinically for efficacy by assessing their ability to protect nonhuman primates against cell-free simian immunodeficiency virus (SIV) or simian-human immunodeficiency virus (SHIV) challenges. However, it remains unclear whether cell-associated virus (virus-infected donor mononuclear cells), cell-free virus, or both play the most important roles in initiating mucosal infection by HIV-1 (1-5). This distinction is critical, since effective strategies for blocking cell-free and cell-associated virus transmission may be very different (3, 6, 7). We sought to explore early events in mucosal transmission of HIV-1 and SIV by evaluating the relative efficiency of cell-associated and cell-free virus in initiating mucosal infection. To model these infection events, we developed a novel three-dimensional sealed human colonic mucosa explant system. We utilized this system in association with the SIV distal colon in vivo challenge model in rhesus macaques to evaluate the relative efficiency of initiating mucosal infection using cell-associated virus compared to that of initiating mucosal infection using cell-free virus in vivo. MATERIALS AND METHODS Viruses.A replication-competent CC chemokine receptor type 5 (CCR5)-tropic HIV-1 strain expressing green fluorescence protein (GFP) [HIV-1(R5) NL4.3-BaL-GFP] (8) was utilized for human organ infections, and SIVmac251 (9) was utilized for rhesus monkey tissue infections.TCID 50 for cell-associated virus and cell-free virus. The 50% tissue culture infective dose (TCID 50 ) was determined as previously described (10). Briefly, 4 replicates each of cell-associated virus (starting with 200,000 cells/well) and cell-free virus (starting with concentrated virus) were added to the first column of a 96-well plate. Then, 5-fold dilutions were performed for a total of 9 serial dilutions (1:5 to 1:5 9 ). An additional column with no virus or cells added served as a negative control to measure the background. TZM-bl cells (NIH AIDS Research and Reference Reagent Pr...

SUMMARY The co-engagement of fragment crystallizable (Fc) gamma receptors (FcγRs) with the Fc region of recombinant immunoglobulin monoclonal antibodies (mAbs) and its contribution to therapeutic activity has been extensively studied. For example, Fc-FcγR interactions have been shown to be important for mAb-directed effector cell activities, as well as mAb-dependent forward signaling into target cells via receptor clustering. Here we identify a function of mAbs targeting T cell-expressed antigens that involves FcγR co-engagement on antigen-presenting cells (APCs). In the case of mAbs targeting CTLA-4 and TIGIT, the interaction with FcγR on APCs enhanced antigen-specific T cell responses and tumoricidal activity. This mechanism extended to an anti-CD45RB mAb, which led to FcγR-dependent regulatory T cell expansion in mice.

To date, most therapeutic and vaccine candidates for human immunodeficiency virus type 1 (HIV-1) are evaluated preclinically for efficacy against cell-free viral challenges. However, cell-associated HIV-1 is suggested to be a major contributor to sexual transmission by mucosal routes. To determine if neutralizing antibodies or inhibitors block cell-free and cell-associated virus transmission of diverse HIV-1 strains with different efficiencies, we tested 12 different antibodies and five inhibitors against four green fluorescent protein (GFP)-labeled HIV-1 envelope (Env) variants from transmitted/founder (T/F) or chronic infection isolates. We evaluated antibody/inhibitor-mediated virus neutralization using either TZM-bl target cells, in which infectivity was determined by virus-driven luciferase expression, or A3R5 lymphoblastoid target cells, in which infectivity was evaluated by GFP expression. In both the TZM-bl and A3R5 assays, cell-free virus or infected CD4؉ lymphocytes were used as targets for neutralization. We further hypothesized that the combined use of specific neutralizing antibodies targeting HIV-1 Env would more effectively prevent cell-associated virus transmission than the use of individual antibodies. The tested antibody combinations included two gp120-directed antibodies, VRC01 and PG9, or VRC01 with the gp41-directed antibody 10E8. Our results demonstrated that cell-associated virus was less sensitive to neutralizing antibodies and inhibitors, particularly using the A3R5 neutralization assay, and the potencies of these neutralizing agents differed among Env variants. A combination of different neutralizing antibodies that target specific sites on gp120 led to a significant reduction in cell-associated virus transmission. These assays will help identify ideal combinations of broadly neutralizing antibodies to use for passive preventive antibody administration and further characterize targets for the most effective neutralizing antibodies/inhibitors. IMPORTANCEPrevention of the transmission of human immunodeficiency virus type 1 (HIV-1) remains a prominent goal of HIV research. The relative contribution of HIV-1 within an infected cell versus cell-free HIV-1 to virus transmission remains debated. It has been suggested that cell-associated virus is more efficient at transmitting HIV-1 and more difficult to neutralize than cell-free virus. Several broadly neutralizing antibodies and retroviral inhibitors are currently being studied as potential therapies against HIV-1 transmission. The present study demonstrates a decrease in neutralizing antibody and inhibitor efficiencies against cellassociated compared to cell-free HIV-1 transmission among different strains of HIV-1. We also observed a significant reduction in virus transmission using a combination of two different neutralizing antibodies that target specific sites on the outermost region of HIV-1, the virus envelope. Therefore, our findings support the use of antibody combinations against both cell-free and cell-associated virus in fu...