I’ve been searching for why most is R5 initially

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The vast majority (up to 90%) of newly transmitted HIV uses the CCR5 coreceptor. R5 virus, also known as M-tropic virus due to its ability to infect macrophages, is more likely transmitted sexually due to the fact that the virus can infect CCR5-expressing macrophages and dendritic cells on mucous membranes of the genital and gastrointestinal (GALT) tract, which carry HIV to the regional lymph nodes and facilitate contact with and infection of activated T helper cells.

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LCs express CD4 and HIV chemokine coreceptors, and these molecules have been shown to mediate infection of LCs (11–14). Immature LCs express surface CCR5, but not surface CXCR4, immediately after isolation from skin; CCR5 on LCs also mediates fusion with cells expressing the HIV envelope protein gp120 (11). Likewise, Patterson et al. (15) examined human cervix and found that CCR5 mRNA expression was at least 10-fold higher than CXCR4 mRNA expression in this tissue. These reports suggest that the HIV coreceptor expression pattern on LCs, or perhaps on other resident cells in genital tissue, confers a “gatekeeper” status on these cells at mucosal surfaces, allowing preferential entry of R5 HIV, but not X4 HIV.

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