Initial evidence that cell contact mediated transmission of
Initial evidence that cell-contact-mediated transmission of HIV-1 is relevant for the generation of latently infected Angiotensin I (human, mouse, rat) was suggested in the context of transmission from dendritic cells to resting CD4+ T cells (Evans et al., 2013, Kumar et al., 2015). As dendritic cells probe for antigens, they capture and retain full particles largely in a CD169-dependent fashion (Izquierdo-Useros et al., 2012, Puryear et al., 2013, Sewald et al., 2015). As they interact with resting CD4+ T cells during antigen presentation, live particles are transmitted to target cells (McDonald, 2010). In addition, cell-cell contact and the tissue microenvironment also help overcome cell resistance to infection and influence the regulation of target cell infection (Shen et al., 2013). Our study explores infection of resting CD4+ cells by cell-to-cell transmission between T cells. These interactions are fundamentally different than interactions involving dendritic cells or endothelial cells. T cells typically do not engage in direct antigen presentation with each other; thus, signaling through the T cell receptors and co-stimulatory receptors is unlikely to be antigen specific (Len et al., 2017). Therefore, the set of signals involved in the process of transmitting HIV-1 to resting cells in the context of T cell-T cell interactions is likely unique. It is possible that the specific signals and environment created during the transmission of HIV-1 to resting CD4+ T cells from activated T cells will affect the establishment of inducible latent infection. This lack of robust T cell signaling, as in the context of antigen presentation, likely prevents productive infection and directs proviruses toward a latent state. Our results suggest that proviruses generated by cell-cell contact between T cells may be harder to induce compared to proviruses generated by cell-free infection. This could be a reflection of the route through which latent infection was generated, the phenotype of cells that harbor latent proviruses, differences in the proportion of defective proviruses, or the presence of multiple proviruses in a small number of infected cells (Chavez et al., 2015, Maldarelli, 2016, Rezaei et al., 2018, Tsunetsugu-Yokota et al., 2016). Nevertheless, our initial observation that latent infection generated by cell-to-cell transmission is not easily reversible requires further investigation, as current approaches to purging the latent reservoir may not be effective in this context. Our proposed model could be optimized for testing this possibility and could help in the search for novel compounds that are more efficient at purging latent provirus.
Acknowledgments This work was funded by the amfAR Mathilde Krim Fellowship in Basic Biomedical Research (award 109263-59-RKRL) and the Immunology Training Grant T32-AI007309. Additional funding was provided by the NIH (awards AI084096, AI097117, and AI118682), the Inflammation Training Grant T32-AI89673-5, and the Providence-Boston Center for AIDS Research (CFAR) (P30-AI042853). We thank Chi Chan and David Levy for advice on transfections. We thank Nathan Roy, Caitlin Miller, Hisashi Akiyama, and Suryaram Gummuluru for advice on imaging cell-cell contacts and reagents. We thank Brian Tilton and the Boston University School of Medicine Flow Cytometry Core for support with cell sorting and flow cytometry analysis. We thank Michael Kirber and the Boston University School of Medicine Cellular Imaging Core for microscopy support.
Introduction Over the last decade, adoptive immunotherapy with T-cell receptor (TCR) engineered T cells came more and more into focus as a promising therapeutic option for the treatment of relapsed or refractory acute leukemias (Morris and Stauss, 2016). In this context, most investigators and clinical trials have focused on the redirection of CD8 T cells with tumor-antigen specific TCRs isolated from HLA-class I restricted CD8 T cells (Fesnak et al., 2016). However, there is an increasing evidence that also CD4 T cells play a key role in anti-tumor immunity and that the adoptive transfer of both CD4 and CD8 subsets induces efficient antitumor responses (Perez-Diez et al., 2007). It has also been shown that CD4 T cells are not only restricted to their helper T cell fate but also mediate cytotoxicity against leukemia cells (Herr et al., 2017, Stevanovic et al., 2012). Therefore, it would be attractive to transfer HLA-class I-restricted TCRs not only into CD8 but also into CD4 T cells to redirect both subtypes to interact synergistically against the same tumor target (Kuball et al., 2005). Since HLA-class I molecules are widely expressed in most tissues, HLA-class I restricted TCRs can induce unwanted on-target toxicity. In contrast, HLA-class II restricted TCRs isolated from leukemia-reactive CD4 T cells might exert less toxicity after transfer in CD4 and CD8 T cells as the expression of HLA-class II molecules is largely restricted to cells of hematopoietic origin. Nevertheless, a prerequisite for both strategies is the availability of TCRs that induce full T-cell activation without contribution of the CD4 or CD8 co-receptor and so far, only few TCRs are characterized by a co-receptor independent function (Kuball et al., 2005, Thomas et al., 2015).