Background:  The exact means of perinatal HIV transmission in utero and intrapartum is unknown. We performed a molecular study to identify possible sources and mechanisms of perinatal HIV transmission.

Methods:  We isolated HIV from a variety of maternal and infant sources during gestation, at delivery, and over the first 12 weeks after birth from 105 infected mother-infant pairs followed prospectively in UCLA’s Los Angeles Pediatric AIDS Cohort between 1989 and 1995. HIV was assayed for by culture and PCR in maternal PBMC, cervicovaginal secretions, and autologous neutralizing antibody assays as well as in infant gastric aspirates collected at birth, infant birth peripheral blood mononuclear cells (PBMC) and infant PBMC collected biweekly over the first 12 weeks of life. HIV env gene region DNA from 3 positive transmitting mother-infant pairs were amplified by PCR, cloned and sequenced. Maternal HIV sequences were aligned with their respective infant’s HIV env sequences and compared by maximum likelihood and neighbor-joining analysis.  

Results:  Of 105 HIV infected mothers, 5 transmitted HIV to their infants. We isolated and sequenced HIV from all sources tested in 3 of 5 infected mother-infant pairs. Phylogenetic analysis indicated that among 2 infants infected in utero, the closest match between the infant’s PBMC virus at birth and the possible virus sources studied was a unique virus present in the gastric aspirate. The viruses present in these in utero infected babies’ time of delivery gastric aspirates although unique, were found to be related to their mother’s neutralization escape variant. The closest source of virus in the infant infected intrapartum was an HIV maternal neutralization escape variant found in maternal PBMC, maternal cervicovaginal secretions and infant gastric aspirate.

Conclusions:  Phylogenetic analysis of 2 in utero transmitting pairs indicated transmission of a unique HIV variant found in birth gastric aspirate samples. This suggests the infants were exposed to these variants by swallowing amniotic fluid prior to delivery. In contrast, phylogenetic analysis of 1 intrapartum transmitting pair identified the infecting virus as a strain found in maternal PBMC, maternal cervicovaginal secretions and infant gastric aspirate suggesting exposure by swallowing blood or cervicovaginal secretions during delivery. Our study indicates that multiple sources of maternal HIV exist and that virus contact with infant mucosal surfaces may be a major mechanism of perinatal transmission both in utero and intrapartum.

Keywords: perinatal transmission; DNA sequencing; amniotic fluid