Background
X4-tropic viruses typically emerge in the late clinical stages of HIV, when the immune system is significantly impaired. As few as two amino acids changes can convert an R5-Tropic virus to an X4-Tropic virus. HIV has an error-prone reverse transcriptase and in the absence of therapy, spontaneous emergence of X4 tropic viruses is expected unless they were specifically restricted. We sought to understand what, if any, immune component was responsible for selective restriction of X4-tropic viruses.
Methods
Viral constructs were generated with an NL4-3 consensus background differing only in the envelope which confers tropism. We utilized the MISTRG 6-15 mouse model either transfused with primary CD4+ T cells to compare X4 and R5 tropic viral fitness in the absence of other immune components, or engrafted with CD34+ cells and reconstituted with a human immune system to assess immune mediated restriction of X4-tropic viruses. Using CRISPR Cas9 targeting the type I interferon (IFN) receptor we generated IFNAR KO humanized mice to assess the contribution of type I IFN signaling. Primary CD4+ T cells were cultured to quantify EC50s for type I IFN for both X4 and R5 tropic viruses. Primary plasmacytoid dendritic cells (pDCs) were cultured with various viral constructs and inhibitors to determine key components for IFN release.
Results
Using the engrafted MISTRG-6-15 humanized mouse model, we observed R5 dominance in mice infected with both viral tropisms, whereas an X4 dominance was observed in mice only transfused with primary CD4+ T cells, suggesting an innate arm of immunity was responsible for the tropism-specific suppression. Plasma cytokine analysis revealed that only R5 tropic viruses triggered detectable type I IFN levels in infected mice. Furthermore, primary pDCs displayed differential type I IFN release in the presence of R5 and X4 tropic viruses ex vivo. Humanized IFNAR KO MISTRG 6-15 mice infected with a mixture of X4 tropic and R5 tropic viruses lost selective X4 tropic restriction. As a consequence, mice with increased proportions of X4 tropic viruses displayed more rapid CD4 T cell decline.
Conclusions
Type I interferon signaling is responsible for selective X4-tropic viral suppression, and this signaling is driven by R5 tropic viruses. This underscores a novel evolutionary tradeoff whereby HIV allows for selective elimination of a subset of viruses, X4-tropic viruses, which would if unchecked result in rapid CD4 decline and host death.