Tryptophan catabolism and immune activation in primary and chronic HIV infection

Kynurenine/Tryptophan ratio (KTR) is increased in HIV infection, and linked to immune activation. We hypothesized that early cART initiation results in lower KTR compared to late initiation. Furthermore, we hypothesized that KTR prior to cART is a predictor of the magnitude of subsequent reduction in immune activation.

Prospective study including 57 HIV-infected individuals (primary HIV infection (N = 14), early presenters (>350 CD4+ T cells/μL, N = 24), late presenters (<200 CD4+ T cells/μL, N = 19)). Kynurenine and tryptophan were analysed by liquid chromatography-tandem mass spectrometry. Total CD4+ and CD8+ T cells were determined and proportion of activated CD38 + HLA-DR+ Tcells was measured using flow cytometry at baseline and after 6 and 12 months of cART.

At baseline, primary HIV infection had higher KTR than early presenters. However, similar KTR in primary HIV infection and early presenters was found after cART initiation, while late presenters had higher KTR at all time points. In primary HIV infection and early presenters, KTR was positively associated with proportion of activated cells at baseline. Furthermore, in early presenters the KTR at baseline was associated with proportion of activated cells after 6 and 12 months. Interestingly, in primary HIV infection the KTR at baseline was positively associated with reduction in proportion of CD8 + CD38 + HLA-DR T cells after 6 and 12 months.

Lower kynurenine/tryptophan ratio during follow-up was found after early initiation of cART. KTR in primary HIV infection and early presenters was positively associated with immune activation. Importantly, KTR in primary HIV infection predicted the magnitude of subsequent reduction in immune activation. Thus, a beneficial effect of early cART on KTR was suggested.