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Background:  We previously identified a primary HIV-1 envelope (Env) obtained at transmission from a rapid disease progressor that showed a distinct ability to allow replication and cytopathicity in the human thymus models. In this work, we dissect the Env regions and elucidate the envelope properties which cause thymic destruction.

Methods:  We determined the relative cytopathicity, receptor affinity, co-receptor affinity, and stability of virus expressing the uniquely pathogenic Env (R3A) and the less pathogenic Env (R3B) isolated from the same patient using in vitro T-cell models. We created R3A and R3B recombinant Env to map the region(s) responsible for each in vitro phenotype. Finally, we created recombinant viruses to test the contribution of putative envelope domains to the pathogenic phenotype in the human fetal-thymus organ culture.

Results:  We found that the pathogenic R3A env is capable of using very low levels of CXCR4 for entry relative to the NL4-3 or R3B Env. Furthermore, R3A is more cytopathic for T cells when expressed in cis or in trans. R3A and R3B expressing virions show the same inherent stability and Env incorporation. Interestingly, the enhanced fusion/entry of R3A mapped to the V1/V2 region while both V1/V2 and V5-gp41 contributed to R3A Env-mediated cytopathicity in vitro. Enhanced affinity for CXCR4 in vitro proved to be a complex phenotype not attributable to one region of the Env. Surprisingly, when the recombinant envelopes were tested in the thymus, V1/V2 was not found to play a role in thymic replication or pathogenesis. Instead, V5-gp41 was able to partially account for the success of R3A, as both R3A (R3B V5-gp41) and R3B (R3A V5-gp41) showed intermediate levels of pathogenesis.

Conclusions:  The V5-gp41 region of the R3A Env is partially responsible for the unique thymic pathogenesis of the R3A Env. Furthermore, in spite of the clear contribution of V1/V2 to R3A entry in vitro, this property has no relevance for pathogenesis in the context of the thymic microenvironment. These data highlight the disconnection between in vitro and in vivo cytopathicity and point to a unique contribution of V5-gp41 to thymic pathogenesis through an as-yet unresolved mechanism.

Keywords: envelope; thymus; pathogenesis