Background: Tissue-resident memory T cells (TRM) are a recently described subset of memory T cells present in tissues. Although memory CD4+ T cells are a key target for HIV infection and the major cellular reservoir for HIV in peripheral blood, the contribution of CD4+ TRM to HIV infection and persistence is not known. Currently, studies of TRM in humans are limited to biopsied and cadaveric tissue; TRM do not recirculate and are virtually absent from peripheral blood. Therefore, despite their potential importance to HIV persistence, the effect of HIV infection on the TRM compartment has not been evaluated. To address this critical need, we utilized BLT humanized mice to investigate HIV infection of TRMin vivo and establish their contribution to HIV persistence.
Methods: We performed a flow cytometric analysis of peripheral blood and cells isolated from the lymphoid (spleen, lymph nodes, and bone marrow) and non-lymphoid tissues (liver and lung) of uninfected BLT mice and BLT mice infected with a CCR5 tropic HIV-1 strain (JR-CSF, CH040 or THRO). Human memory T cells (CD3+CD45RO+) that expressed CD69 were classified as tissue-resident (TRM). CD25 and HLA-DR negative TRM were classified as resting. TRM were isolated from the tissues of HIV-infected BLT mice by FACS and HIV-DNA and HIV-RNA quantitated by real-time PCR.
Results: As observed in humans, TRM were present in lymphoid and non-lymphoid tissues of BLT mice but virtually absent from peripheral blood (p<0.0001). Resting CD4+ TRM were also present in lymphoid and non-lymphoid tissues of BLT mice (mean % of CD4+ TRM= spleen: 27%, lymph node: 61%, bone marrow: 28%, liver: 35%, lung: 33%). During HIV infection, CD4+ memory T cells were significantly depleted from the tissues of BLT mice (p=0.0002). CD4+ TRM were also significantly decreased in HIV-infected BLT mice (p=0.0003) and preferentially depleted from non-lymphoid tissues (>85% decrease) in comparison to lymphoid tissues (27-55% decrease). Quantitative real-time PCR demonstrated the presence of HIV-DNA and HIV-RNA in CD4+ TRM of all tissues analyzed from HIV-infected BLT mice (mean: 3X104 HIV-DNA and 1X105 HIV-RNA copies per 106 cells).
Conclusions: Human TRM are present in BLT mice and their tissue distribution recapitulates the human condition. Importantly, HIV infects and depletes CD4+ TRM in tissues. In addition, TRM contain significant levels of viral DNA and produce high levels of viral RNA. Currently, we are evaluating the contribution of CD4+ TRM to the latent HIV reservoir in vivo.