HTLV-1A, the most common HTLV-1 subtype, and HTLV-1C, the most divergent type, differ substantially. The apparent differences in susceptibility to infection and disease manifestation suggest that the 2 viral subtypes may affect their host's immune system differently. HTLV-1A and C have similar genetic organization: the structural Gag and Envelope proteins, viral enzymes, Reverse Transcriptase, Integrase and Protease, and the transcriptional and post-transcriptional regulators Tax and Rex proteins are highly conserved between the 2 viral subtypes. Strikingly, however, the RNA sequence of orf-I and orf-II, encoding the p12, p8, and p30 proteins, are highly divergent in HTLV-1C. The proteins encoded by HTLV-1A orf-I and orf-II are essential for HTLV-1A to persist in the host and escape immune recognition. The p8/p12 proteins target MHC-I and ICAM-1 expression and inibit the killing of infected cells by Cytotoxic T cells, Natural Killer cells. The p30 protein, via the PU.1 transcription machinery, inhibits interferon responses in monocytes. It is thus conceivable that the expression of these proteins in T cells in individuals with a high number of infected cells may affect the overall host response to pathogens. We therefore focused on orf-I to investigate potential biological differences between HTLV-1A and C. In HTLV-1C, we found that a putative doubly-spliced mRNA produces a 16KD protein (p16) by juxtaposing the first exon of Rex in frame with orf-I. We found that p16 inhibits inflammasome activation and cytokine production, an essential step in monocytes to respond to pathogens. We constructed chimeric HTLV-1A/C viruses whereby either the entire 3′ end or the Orf-I of HTLV-1C was swapped into the HTLV-1A backbone. The overall aim of our studies is to investigate the effect of the orf-I proteins of both viral subtypes on immune cells in vitro and in vivo using isogenic HTLV-1A/C chimeric viruses orf-I mutants.
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