TGF-β1 enhances R5-tropic HIV-1 infection in activated and resting memory CD4+T cells by upregulating CCR5

Abstract

TGF-β1 (or TGF-β below), a pleiotropic cytokine with potent immune suppressive activity, was found to be highly upregulated in HIV-1-infected individuals. Given that the major pathogenesis of HIV-1 infection is the progressive loss of CD4+ T cells and acquired immunodeficiency syndrome (AIDS), it was previously indicated that TGF-β contributes to the HIV-1 pathogenesis by immunosuppression of different immune cells, induction of regulatory T cells (Tregs), and fibrosis in lymphoid tissues. More importantly, TGF-β was suggested to be a biomarker of HIV-1 disease progression (i.e. TGF-β increases with the severity of disease). I, therefore, hypothesize that the underlying mechanism of TGF-β-mediated pathogenesis is associated with the functional regulation of HIV-1 target CD4+ T cells. In this study, I found that TGF-β significantly upregulates chemokine receptor CCR5 expression in both activated and resting memory CD4+ T cells. SB431542, the inhibitor for TGF-β type I receptor kinase, can significantly block the TGF-β-mediated upregulation of CCR5. Furthermore, the TGF-β-mediated upregulation of CCR5 is dependent on Smad3 phosphorylation and is likely through post-transcriptional regulation. Based on these results, I further hypothesize that TGF-β may increase CCR5-tropic HIV-1 infection to memory CD4+ T cells because CCR5 is the key co-receptor of HIV-1 entry. Indeed, using both pseudovirus and live virus to measure the infection susceptibility, a significant increase was found after TGF-β treatment in both activated and resting memory CD4+ T cells while SB431542 can significantly block the TGF-β-mediated increase in HIV-1 infection susceptibility. This result suggests a novel pathogenic pathway that TGF-β can directly enhance HIV-1 infection in memory CD4+ T cells by the upregulation of CCR5 expression.

More importantly, resting memory CD4+ T cells are recognized as a major reservoir for HIV-1 latency and at the same time displaying strong restriction to HIV-1 infection. The increase in HIV-1 infection in resting memory CD4+ T cells provides a potential role of TGF-β in enhancing the HIV-1 latency formation. To test this hypothesis in vivo, I explore the use of humanized mice model to test the effect of TGF-β in HIV-1 latency formation. I found that the human PBMC engrafted NSG mice (NSG-HuPBL) is adaptable to the cART drug treatment and successfully shows viral rebound after the cessation of cART. This model, however, is unlikely suitable to determine peripheral latency because no resting CD4+ T cells can be detected in blood samples. In contrast, peripheral resting CD4+ T cells are readily detected in the human CD34+ hematopoietic stem cells engrafted NSG mice (NSG-HuCD34+), providing a potential murine model for HIV-1 latency study.

Finally, I found that semen plasma contains extraordinarily high level of TGF-β compared to other body fluids. I then hypothesize that seminal TGF-β may enhance HIV-1 infection during sexual transmission by upregulating CCR5 expression in mucosal CD4+ T cells of healthy individual. In vitro experiments, however, showed that the seminal plasma contains mainly inactive forms of TGF-β with high cytotoxicity. Future experiments should explore the activation of TGF-β by low pH vaginal fluids and the role of subsequently activated TGF-β in HIV-1 sexual transmission.