miR-155 is differentially expressed in Treg subsets, which may explain expression level differences of miR-155 in HIV-1 infected patients

To the editor:

We read with interest the article by Bignami et al that examined the miRNA profile of CD4⁺ T lymphocytes from patients infected with HIV-1 (long-term non-progressors or treatment naive) versus subjects multiply exposed but uninfected (MEU).¹  One of the main findings was the differential expression of only a single miRNA, miR-155, between LTNP and naive HIV-1 infected patients whereas there were many more differentially expressed miRNAs noted between HIV-1 infected patients and MEU. It was suggested that the up-regulation of miR-155 in naive HIV-1 infected patients may contribute to pathogenesis.

miR-155 has previously been noted to play a role in the development of T regulatory cells (Tregs) in murine models.²  We separated out naive Tregs (nveTregs), effector/memory Tregs (eff/memTregs), and conventional naive CD4⁺ T cells (nveCD4⁺) from healthy donors based on the differential expression of CD62L, CD45RO, CD25 and CD127 and measured miR-155 levels in these subsets. Tregs were separated using established protocols³  and were: nveTregs (CD25⁺CD127^lowCD62L⁺CD45RO⁻, eff/memTregs (CD25⁺CD127^lowCD62L^+/−CD45RO⁺) and nveCD4⁺ (CD25⁻ CD127^highCD62L⁺CD45RO⁻). We found miR-155 was strongly expressed in eff/memTregs compared with lower expression levels in both nveTregs and nve CD4⁺ subsets (Figure 1). Further, we found that levels of eff/memTregs are significantly increased in HIV-1 infected individuals with progressive infection (n = 12) versus those who are LTNPs (n = 9; P = .009 and data not shown), confirming previously reported observations.⁴  The higher level of activated Tregs with presumably increased miR-155 levels may explain why these levels were different between the 2 HIV-1 infected groups. It was noted in the article that Tregs from naive patients had higher circulating levels of HLA-DR (used as a marker of Treg activation) and we hypothesize that if miR-155 levels were examined in Treg subsets from LTNP versus naive that there would be a difference in miR-155 levels between activated Tregs and both non-activated Tregs and conventional CD4⁺ subsets and it may be the cell numbers within these sub-populations that explains why levels were noted to be higher in the naive population. Recently, the miRNA profiles from several lymphocyte subsets have been shown to reveal distinct miRNA signatures,⁵  which further adds to the argument that differences in T cell subsets in HIV-1 infection may be a cause of why there are differences in miRNA levels between various HIV-1 infected populations.

In addition, checking additional time points for each patient would be a reasonable next step to establish if the miRNA profile from CD4⁺ T lymphocytes is stable and can be used to distinguish HIV-1 infected, MEU and healthy donors. As a first step, we agree that examining miRNA levels extracted from total CD4⁺ T cells is a logical and practical place to start but as we start to understand the nuances of miRNA expression levels in T cell subsets, the future would suggest that more information will be gained by studying miRNA signatures within individual T cell subsets.