HTLV-1 Proviral Load in Vaginal Fluid Correlates With Levels in Peripheral Blood Mononuclear Cells

Background: The prevalence of human T-lymphotropic virus type-1 (HTLV-1) infection is higher in women, and sexual intercourse has been described as an important route of male-to-female transmission. The present study aimed to quantify HTLV-1 proviral load (PVL) in vaginal fluid, and to investigate correlations with PVL in peripheral blood mononuclear cells (PBMC). Methods: HTLV-1-infected women were consecutively recruited at a multidisciplinary center for HTLV patients in Salvador-Brazil. All women underwent gynecological examinations to obtain cervicovaginal fluid and venipuncture for blood collection. PVL, as measured by real-time quantitative polymerase chain reaction (RT-qPCR), was expressed as the number of copies of HTLV-1/106 cells in blood and vaginal fluid samples. Light microscopy was used to assess cervicovaginal cytopathology and vaginal microbiota. Results: In the 56 included women (43 asymptomatic carriers and 13 diagnosed with HTLV-associated myelopathy/tropical spastic paraparesis-HAM/TSP), mean age was 35.9 (SD±7.2) years. PVL was higher in PBMCs [median: 23,264 copies/106 cells (IQR: 7,420 - 64,371)] than in vaginal fluid [451.9 copies/106 cells (IQR: 0 - 2,672)] (p<0.0001). PVL in PBMCs was observed to correlate directly with PVL in vaginal fluid (R=0.32, p=0.01). PVL was undetectable in vaginal fluid samples from 20 women (35.7%), most of whom were asymptomatic (19/20). Cytopathologic analyses revealed no differences between women with detectable or undetectable PVL. Conclusion: HTLV-1 proviral load is detectable in vaginal fluid and correlates directly with proviral load in peripheral blood. This finding suggests that sexual transmission of HTLV-1 from female to male may occur and vertical transmission, particularly in the context of vaginal delivery.