HIV-1 infection during pregnancy and in children : significance of HIV-1 variability and the placental barrier

Abstract

With the global increase in human immunodeficiency virus 1 (HIV-1) infection in women of childbearing age, there has also been an alarming increase in the number of mother-to-child transmissions of HIV-1. Although antiretroviral therapy and Cesarian section have been demonstrated to significantly decrease the vertical transmission rate of, these interventions are not widely available in the developing world. Therefore, studies of the mechanisms of vertical transmission are important.

This thesis was based on a prospective follow-up of HIV-1-infected patients, especially mothers and children in Sweden and in Cameroon. Coreceptor usage of the primary HIV-1 isolates was determined on indicator cell lines U87.CD4 and/or GHOST(3) engineered to express CD4 and one of the main chemokine receptors (CCR1, CCR2b, CCR3, CCR5, CXCR4, Bob and Bonzo). The genetic subtype of the viruses was determined by phylogenetic analysis of DNA sequences or by heteroduplex mobility assay (HMA).

Coreceptor usage of 81 primary HIV-1 isolates was analysed in relation to their biological phenotype, the genetic subtype and the severity of HIV-1 infection in adult patients. In all cases, tropism for MT-2 cells, a previously used indicator cell line, correlated with CXCR4 usage. Importantly, we found subtype-dependent differences in coreceptor usage, in that dual tropic R5M viruses were detected less frequently among subtype D isolates and that CXCR4 use was less frequent among subtype C isolates.

Coreceptor usage of primary HIV-1 isolates obtained from Cameroonian pregnant women was also tested. HIV-1 mother-to-child transmission rate was 11.9%. The majority of the virus isolates were of envelope subtype A and used CCR5 as coreceptor for cell entry. Four isolates of 28 (14.2%) were able to use the orphan receptor Bonzo as well. In two mothers (of four tested) who transmitted HIV-1 to their children, virus with CCR5 Bonzo dual tropism emerged during pregnancy. Paired isolates from mother and child were available in two cases and the coreceptor usage of these pairs was concordant. Further analysis of available sequences from env and pol showed that a majority of virus variants were recombinants including subtype A. We found CRF02_AG(IbNG), a previously unrecognized A/J recombinant and even more complex recombination patterns.

We studied HIV-1 infected mothers and their children living in Sweden, where five genetic subtypes were represented. Regardless of genetic subtype, HIV-1 isolated during pregnancy used in most cases CCR5. Interestingly, all children carried virus of RS phenotype at the time of HIV-1 diagnosis. However, in the children born to mothers that carried X4 virus a change in HIV-1 coreceptor use could be documented several months later. The emergence of such viruses was preceeded by a drop in CD4 +T cell counts and clinical progression in all patients.

To test the placental barrier function, we obtained ten term placentae from a cohort of pregnant women in Sweden, identified as infected by HIV-1 subtype A, B or C or HIV-2. All mothers underwent antiretroviral therapy and all children were uninfected. HIV sequences were detected by PCR in mother's PBMC and in enriched placental trophoblastic cell preparations. After immunomagnetic cell separation of these mixed populations, the purified trophoblasts were overall negative. Proviral DNA was found in the non-trophoblastic placental cells, mainly T-lymphocytes, which were shown to be a mixture of maternal and foetal. cells. This study indicated that the placental barrier, Le., the trophoblastic layer, was not HIV infected.