SIVsm infection in mangabeys leads to an early increase in T-cell proliferation. T-cell proliferation determined by percentage of Ki67⁺ staining is presented for CD4⁺ (A) and CD8⁺ (B) T cells. CD4⁺ and CD8⁺ T-cell proliferation was sharply increased after SIVsm infection and generally remained elevated throughout 37 weeks of infection. Longitudinal data are shown for 6 mangabeys: 3 with thymuses (continuous lines; SM1, blue, □; SM2, turquoise, ▵; SM3, purple, ▿) and 3 without thymuses (broken lines; SM4, green, ▴; SM5, black, •; SM6, red, ▪).