The role of trophoblast cells in HIV-1 passage across the placenta.

Abstract

Background The combination of pre-eclampsia and human immunodeficiency virus (HIV) remains a major obstetric dilemma in South Africa and, consequently, mother to child transmission of HIV remains a burden in our community. The human placenta is in direct contact with maternal blood and is therefore susceptible to HIV infection or transmission. Cluster of differentiation (CD) 4 (CD4) and C-C chemokine receptor type 5 (CCR5 or CD195) are known to be important receptors for HIV transmission. Maternal in utero transmission is thought to mainly rely on R5-trophic viruses. Intercellular adhesion molecule 2 (ICAM-2 or CD102) is a member of intercellular receptor family found on endothelium and leucocytes that is involved in cell mediated immune recruitment. ICAM-2 is implicated in HIV transmission. The aim of this study was to immunohistochemically localise HIV (p24 viral core) and the HIV receptor (CD4) within the placenta of HIV associated pre-eclamptic pregnancies to elicit the mechanism of vertical transmission of HIV. A further aim was to immuno-localise the HIV co-receptor (CCR5) and ICAM-2 within these placentae. Method Post institutional ethics approval, a retrospective cohort of 80 out 180 archived paraffin embedded placental blocks was sourced from mothers who delivered at Prince Mshiyeni Memorial district hospital in KwaZulu-Natal. Four groups (n=20/group) were categorised according to pregnancy status (normotensive and pre-eclamptic) and HIV status (positive and negative). Pre-eclampsia was defined as new onset hypertension (≥140/90mmHg) with proteinuria. Immunohistochemistry was performed using the Envision kit (DAKO, Denmark). Anti-human mouse CD4, and p24 antibodies were used to identify presence of HIV and CD4 receptors. The antihuman mouse antibodies CCR5 and ICAM-2 were used to detect these receptors within placentae. An Axiovision A1 light microscope and Axiovision (Zeiss) software was used for image acquisition and analysis where percentage staining in microns per field area (20x objective) was measured. Results Eight out of 180 HIV positive mothers in both pre-eclampsia and normotensive groups transmitted HIV to their babies (4%) despite receiving antiretroviral therapy. CD4 positive maternal immune cells and endothelial cells lining fetal vessels were found. CD4 was very rarely seen on syncytiotrophoblast. p24, however, was present in both the maternal and fetal blood circulation, as well as within Hofbauer cells. CCR5 showed diffuse staining of all exchange villi within all four cohorts. A factorial univariate ANOVA was then performed and both HIV and pregnancy status had a significant main effect on expression of CCR5 in placental tissue. There was greater expression of CCR5 in the HIV positive groups compared to the HIV negative groups [F (1,169) =6.979, p=0.009] and the pre-eclamptic compared to the normotensive groups [F (1,169) =8.803, p=0.003]. ICAM-2 was sparse and found in areas around syncytial knots as well as lining an arterial supply in a stem villus. An independent samples Mann-Whitney U test showed a significant difference in the distribution of ICAM-2 expression between the pre-eclamptic and normotensive groups (p<0.001) and the HIV positive groups. The HIV negative pre-eclamptic group showed the greatest expression of ICAM-2 compared to the 3 other groups. Discussion and Conclusion The immunolocalisation of p24 provides evidence of HIV successfully traversing the fetomaternal barrier. Interestingly, HIV was found to be present in the placentas of babies that were HIV positive as well as those who were HIV negative 6 weeks later. This suggests an unknown mechanism that protects the fetus from viral insult. All receptors and co-receptors used for HIV infection are found on the trophoblast, suggesting that this barrier is susceptible to HIV infection. Pre-eclampsia increases expression of both cytokine receptors and inflammatory markers which may increase susceptibility to infection. Further ultrastructural and biomolecular work to identify the immune cells and expression of receptors involved will corroborate our findings.