Identification of a Novel HIV-1 Circulating Recombinant Form (CRF181_BC) with a Unique Subtype B Backbone from the China-Myanmar Border Region.

Yunnan Province has historically been a major gateway for the introduction of HIV-1 into China. The border region with northern Myanmar has become a significant hotspot for HIV-1 recombination. This study identified and characterized a novel circulating recombinant form (CRF) of HIV-1 in the area, designating it as CRF181_BC. This viral strain was detected in three cases of heterosexual transmission in Baoshan City, Yunnan Province, China. Phylogenetic analysis of the near-full-length genome revealed that the three sequences formed a distinct branch, separate from all recognized subtypes and CRFs. The recombinant structure comprises 12 alternately distributed B and C subtype fragments, with subtype B accounting for 55.1% of the genome length. This contrasts with other known CRF_BC strains, which typically have a C-subtype backbone. Bayesian analysis revealed that this recombinant virus emerged between 2005 and 2006, coinciding with Yunnan Province's critical transition from HIV transmission dominated by injection drug use to HIV transmission predominantly occurring through sexual contact. The discovery of CRF181_BC underscores the intricate genetic diversity of HIV-1 and ongoing active recombination events along the China-Myanmar border. This poses new challenges for local viral diagnosis, treatment, and vaccine development.