Response: Intracellular restriction factors contribute to susceptibility of monocytes/macrophages to HIV-1 infection

Response:

Swaminathan et al¹  stated that despite the higher levels of anti-HIV microRNAs (miRNAs) in monocytes, there is no demonstration in our paper that the low-level expression of both HIV-1 primary receptor CD4 and coreceptor CCR5 on monocytes is not the more significant factor contributing to monocytes' resistance to infection. They argued that in their experience only a small subset of monocytes (< 5%) express CCR5. This argument, however, has its limitation, given the fact that there is tremendous variability in CCR5 expression on primary monocytes from different donors. Several studies have shown that monocytes express relative high levels of CCR5.^2,3  We pointed out in the paper that although the demonstration of differential expression of CCR5 receptor on monocytes and macrophages provides an explanation for the differential susceptibility of these cells to HIV-1 infection,^4,–6  this observation does not account fully for differences in HIV-1 infectivity in monocytes and macrophages. Peng et al showed there was no substantial difference in the expression of CCR5 in donor-matched monocyte and macrophage populations; in contrast, they found that CD4 and/or CCR5 expression decreased during macrophage maturation, despite increasing susceptibility to HIV-1.⁷  They demonstrated that the expression of intracellur APOBEC3 is linked to susceptibility of monocytes and macrophages to HIV-1 infection.⁷  Their funding in conjunction with ours strongly support the notion that intracellular restriction factors indeed contribute to susceptibility of monocytes/macrophages to HIV-1 infection.

Swaminathan et al compared their preliminary work using CD4⁺ T cells with ours. However, there are significant differences in the study designs between theirs and ours. First, in our study we used monocytes and macrophages isolated from the same donors. In contrast, Swaminathan et al used CD4⁺ T cells from the subjects of 2 different groups. Second, we examined the levels of the anti–HIV-1 miRNAs in donor-matched monocytes and macrophages before HIV-1 infection, whereas Swaminathan et al compared HIV-1–infected CD4⁺ T cells with uninfected cells. As they suggested, it is possible that HIV-1 infection may down-regulate the expression of the anti–HIV-1 miRNAs in CD4⁺ T cells, which explains the difference in the miRNA expression in CD4⁺ T cells from the subjects of 2 groups. Third, in our study we were able to establish the association between the expression of the anti–HIV-1 miRNAs and HIV-1 infectivity in monocytes and macrophages, as we demonstrated that the suppression of the anti–HIV-1 miRNAs in monocytes facilitates HIV-1 infectivity, whereas increase of the miRNA expression in macrophages resulted in the inhibition of HIV-1 replication.

Swaminathan et al also raised the concern about the use of the miRNA inhibitors and their modest effect on HIV-1 reverse transcription (RT) in monocytes. The difference in modulation of HIV-1 infectivity in monocytes and macrophages transfected with the miRNA inhibitors or the miRNAs could be due to the fact that the transfection efficiency differed in monocytes and macrophages. We agree with the suggestion that future studies are necessary to determine whether the miRNA inhibitors have indirect effects on HIV-1 replication through changes in function and phenotype of monocytes and macrophages. Nevertheless, our demonstration that the cellular miRNAs that have anti–HIV-1 ability in CD4⁺ T cells⁸  are also associated with the protection of monocytes and macrophages from HIV-1 infection not only provides an additional explanation to address the question that has puzzled us for almost 2 decades but also offers insight into development of intracellular innate immunity-mediated therapeutics for HIV-1 infection and AIDS.