Background:  HIV is linked to a switch from a Th1 to a Th2 cytokine profile, a shift from a Th1 to Th0 phenotype, and no effect on this cytokine paradigm. These discordant findings may be attributed to indirect experimental strategies.  In this study, we utilized the chemoattractant receptor-homologous (CRTH2) marker to identify Th2⁺ cells and assessed the impact of HIV on the cytokine profile of Th2 cells.

Methods:  To validate CRTH2 as a marker of Th2 cells, CD4⁺ T cells from healthy donors were stimulated with PMA/Ionomycin and immunostained for CRTH2 and intracellular interferon-gamma (IFN-g) and interleukin-4 (IL-4). Phenotypic profile of CD4⁺CRTH2⁺ T cells was determined by expression of CRTH2, CD95, HLA-DR, CD25, CD69, and CD45RO, CD28, and CD27 using 4-color flow cytometry. To evaluate the effect of HIV infection on cytokine expression of Th2 cells, CD4⁺ T cells were mock infected or infected with a primary isolate of HIV. At day 6, the cells were stained for CRTH2 and intracellular IFN-g and IL-4. To evaluate the in vivo association between HIV infection and cytokine expression of CD4⁺CRTH2⁺ T cells, peripheral blood mononuclear cells (PBMCs) were isolated from 7 healthy controls and 13 HIV⁺ patients and stimulated with phytohemagglutinin (PHA); then the frequency of Th0 (IL-4⁺INF-g), Th1 (INF-g⁺IL-4^−­ ) and Th2 (INF-g⁻IL-4⁺) cells was evaluated by conventional flow cytometry.

Results:  CRTH2 expression is confirmed to delineate a Th2 subset as indicated by robust inducible IL-4 response. Approximately, 2 to 4% of CD4⁺ T cells are CRTH2⁺. CD4⁺CRTH2⁺ T cells are inherently more activated than CRTH2⁻ cells, as indicated by higher percentage expression of CD69, CD45RO, CD95, CD25, and HLA-DR. CD4⁺CRTH2⁺ T cells are not terminally differentiated as indicated by expression of CD27 and CD28.  In vitro HIV infection of primary human CD4⁺CRTH2⁺ T cells potently up-regulated IFN-g production while still maintaining robust IL-4 expression. This Th0 (IFN-g⁺ IL-4⁺) phenotype was up-regulated in CD4⁺CRTH2⁺ T cells post-HIV infection by 18-fold, demonstrating a shift to a Th0 phenotype. Ex vivo studies demonstrated that HIV⁺ patients exhibited a decline in CD4⁺CRTH2⁺ cells and a shift of this population toward cells that express both IFN-g and IL-4.

Conclusions:  These data indicate that HIV replication in Th2 cells induces a Th0 phenotype. This phenomenon may be a deliberate viral escape mechanism to prevent the skewing of the immune response towards Th1 or Th2.