Parasite-specific CD8⁺ T cells encounter antigen and express markers of activation during chronic infection.

(A) The frequency (left) and number (right) of CD8+ T cells specific for the immunodominant epitope, TSKB20, in the pool of all CD44+ CD8+ T cells was measured longitudinally in spleen (open square) and skeletal muscle (open triangle). (B) Representative histograms of the KLRG1 expression in the total CD8+ (top) and TSKB20+ (bottom) populations. CD44hi KLRG1+ cells detected in the total (white bar) and parasite-specific (black bar) CD8+ T populations in the spleen and muscle at 150 dpi. (C) Recently activated CD8+ T cells are enriched in skeletal muscle. Representative flow plots show surface expression of CD69 for cells isolated from indicated tissue during chronic (150 dpi) T. cruzi infection. Numbers describe the percentage of cells expressing CD69 in each gate from the total CD44+ population. Summary data of the percentages of CD69+ cells in total (white) and TSKB20+ (black) bars are displayed graphically to the right. (D) Quantification of chronic parasite burden in skeletal muscle via qPCR during chronic (>250 dpi) Brazil and TCC strain T. cruzi infection. The line represents the limit of detection for qPCR. Persistence of the TCC strain has been previously confirmed via qPCR and hemoculture following immunosuppression [86] (E) The frequency of CD8+ T cells that express CD69 in the muscle of mice chronically infected is highest in those infected with Brazil strain relative to the low level but persistent, TCC strain. (F) Reduction in parasite burden following short-term benznidazole treatment results in decreased CD69 expression and contraction of parasite-specific T cells in the muscle during chronic infection (>250 dpi). (G) Antigen-dependent inflammation observed during chronic infection is reduced following sub-curative (19d) treatment with the trypanocidal compound benznidazole (BZ). Skeletal muscle was fixed in 10% formalin, embedded in paraffin, sectioned, and stained with H&E. All data are representative of at least three independent experiments with n = 3–5 with the exception of panels D-G which are representative of two independent experiments. Data are mean + SEM. * indicates percentage levels that are significantly different (* P ≤ 0.05, ** P ≤ 0.01,***P ≤ .001) between specified groups.