Background:  Immune reconstitution inflammatory syndrome (IRIS) is a paradoxical clinical worsening of a subclinical condition during adequate response to ART. IRIS predisposing factors include low pre-ART CD4 counts and robust virological response to ART; however, its pathogenesis remains unclear. We addressed this question studying a group of South African HIV-1-infected subjects presenting with M. tuberculosis (MTB)-associated IRIS.

Methods:  IRIS definition:  ≤3 months on ART; documented viral load and CD4 response to ART, acute symptoms incompatible with normal TB presentation or ART-related adverse events, confirmation of MTB infection. Subjects for the IRIS group were:  17 (9 female, 8 male) HIV⁺ with IRIS presentation (12 pulmonary, 5 extra-pulmonary). Subjects for the control group were:  23 (18 female, 5 male) HIV⁺ with similar ART duration. ART regimens were:  lamivudine (3TC) + stavudine (d4T) + efavirenz (EFV), except 2 IRIS and 1 control, 3TC+d4T+NVP; and 1 IRIS, 3TC + atazanavir (ATV) + tenofovir (TDF). Flow cytometry stainings were performed on whole blood samples using custom-designed lyoplates and analyzed on a 4-color flow cytometer (BD Biosciences). Between-group comparisons were assessed using the Mann-Whitney test; p <0.05 were considered significant.

Results:  Per study design, pre-ART CD4 count, viral load and time on ART were similar in the 2 groups. At the time of IRIS, CD4 and CD8 counts, but not percentage, were lower in the IRIS group. IRIS subjects had lower frequency of CD8⁺ CD28⁺ (but not CD4⁺) and CD8⁺CD25⁺CD62L⁺ T cells, and higher frequency of CD8⁺CD28^- T cells. The frequency of CD4 and CD8 T cell activation (CD38⁺), memory subsets distribution and classic CD4⁺CD25⁺CD62L⁺ T_reg cells was similar in both groups. Compared to the control group, IRIS subjects had significantly higher neutrophil count and lower lymphocyte count, hemoglobin and serum Na⁺ and Cl^–. Levels of immature CD161⁺CD16^–CD56^– natural killer (NK) cells and CD197⁺ mature myeloid dendritic cells (DC) and NK-like CD56⁺ T cells were higher in the IRIS group, whereas mature (CD16⁺) and activated (CD69⁺ or HLA-DR⁺) NK cells were lower than controls. Other NK and DC subsets were similar in the 2 study groups.

Conclusions:  We report that MTB-associated IRIS is characterized, at presentation, by expansion or redistribution of innate immunity effectors, including myeloid cells (neutrophils and myeloid DC), immature NK cells and NK-like T cells, accompanied by lower lymphocyte counts. This expansion of the innate compartment occurs without evidence of generalized T cell activation.