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

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 (PBMCs). In addition, cytopathological alterations and vaginal microbiota were evaluated.

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.

In the 56 included women (43 asymptomatic carriers and 13 diagnosed with HTLV-1-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: 6776-60,036) than in vaginal fluid (451.9 copies/10⁶ cells; IQR: 0-2490) (p < 0.0001). PVL in PBMCs was observed to correlate directly with PVL in vaginal fluid (R = 0.37, p = 0.006). PVL was detected in the vaginal fluid of 24 of 43 (55.8%) asymptomatic women compared to 12 of 13 (92.3%) HAM/TSP patients, p = 0.02. Cytopathologic analyses revealed no differences between women with detectable or undetectable PVL.

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 females to males may occur, as well as vertical transmission, particularly in the context of vaginal delivery.