Research question
What are the virological and immunological determinants of mother-to-child transmission of HIV-1 through breastmilk?
Fit with Africa Centre research portfolio
This programme of work falls under Objective 1 in the Africa Centre research strategy: HIV dynamics – understanding the HIV epidemic.
Data sources used
This work builds on previous research on mother-to-child transmission at the Africa Centre, and utilises stored breastmilk samples from the Wellcome Trust-funded Vertical Transmission Study (VTS), and the detailed data on mothers and infants, including mode of infant feeding and timing of HIV sero-conversion in infants. In most steps all cases of postnatal transmission are compared to non-infected controls, with samples relating to the estimated timing of infection in the cases.
Methods/study design
This programme of work consists of eight investigative steps, involving retrospective analysis of stored breastmilk samples from the VTS, and a final modelling step:
1. RNA and DNA viral load quantification. This addresses the importance of cell-free (RNA) and cell-associated (DNA) viral load quantification in breastmilk of mothers who transmitted HIV postnatally and perinatally to their infants.
2. Amplification and cloning of breastmilk RNA and DNA to identify the origin of transmitted virus as being cell-free or cell-associated.
3. Proteomics/protein profile of breastmilk in transmitting and non-transmitting women. Can breastmilk transmission be identified by specific protein profile in breastmilk?
4. Anti-infectious soluble factors present in breastmilk of transmitting and non-transmitting women. Is the concentration of anti-infectious soluble factors, inflammatory peptides, chemokines and breastmilk enzymes predictive of breastmilk transmission?
5. Humoral immune responses. Is the HIV-specific antibody response in milk protective against breastmilk transmission? Does the presence of neutralising antibodies in the mother prevent postnatal transmission of HIV?
6. Co-infections with milk-borne viruses to examine whether maternal co-infections with milk-borne viruses are involved in HIV-1 transmission in breastmilk.
7. Genetic factors in infants infected postnatally.
8. Examination of viral load and anti-infectious soluble factors in women with breast health problems.
Preliminary findings
Analyses for Step 1 are almost complete. Preliminary findings : Among 37 mother-infants pairs with postnatal transmission, median infant age at HIV acquisition was 94 days [range: 49 to 196]. Cases had lower antenatal CD4 count (median 345 vs. 521 cells/l, p=0.02) and higher antenatal plasma viral load (4.48 vs. 4.15 log copies/ml, p=0.11 ) than controls. Pre-HIV acquisition, cases and controls were similar in breastfeeding patterns (23 vs. 27 exclusively breastfed), median duration of exclusive breastfeeding (61 [0 - 177] vs 70 days [0 - 188]) and child milk intake (mean 730 vs 734 ml/day). Cases were more likely to shed virus in breastmilk than controls: 62% vs 9% always shed, 22% vs 20% shed intermittently and 16% vs 71% never shed, respectively (overall p < 0.001). Among shedders, mean milk viral load was 3.49 log in cases and 2.74 log copies/ml in controls (p=0.003). Cumulative HIV exposure was 15 times higher in cases (19.2x10 7 vs 1.31x10 7 copies, p<0.001). This association persisted among 14 women with CD4 count >350 cells/l (14.9x10 7 vs 1.31x10 7 , p<0.001), although antenatal CD4 cell count (543 vs 569 cells/l, p=0.3), and antenatal viral load (4.2 vs 4.07 log copies/ml, p=0.6) did not differ in cases and controls in this sub-group, due to limited numbers and lack of statistical power.
Laboratory work for Steps 2 and 3 are currently ongoing, and will be undertaken during 2010 for the other steps. During 2011, the results from all steps will be taken together into one complex statistical model of postnatal transmission to identify and quantify the relative contribution of each of the virological and immunological and genetic factors.
Policy implications
The protection offered by breastmilk in relation to HIV transmission is likely to involve a complex relationship between several biological agents. Few studies have analysed such a detailed range of putative factors using the same samples; none have managed to unravel the main determinants of MTCT through breastmilk.