Percentages of (A) naïve (CD28 + CD95-), (B) central memory (CD28 + CD95+) and (C) effector memory (CD28-CD95+) CD3+ T-cells in peripheral blood of both Experimental (EG) and Control group (CG) of macaques. Note that naïve T-cell population showed progressive increase in both groups following immunization. A significant increase in naïve T-cell population was observed in animals from CG group compared to EG after SIV challenge. Following challenge there was increased and decreased expansion of effector and central memory T-cell population respectively in both EG and CGs of animals. Cells were gated through CD3+ “bright” T lymphocytes and at least 20,000 events were collected by gating on lymphocytes. Percentages of respective cell population represent cells out of total lymphocytes. PV and PC denote post vaccination and post challenge time points, in days, respectively. Black arrow shows the time of SIVMAC251 challenge. Asterisk (*) indicates a significant difference between EG (n = 5) and CG (n = 4) for the specified cell subset using the Student’s t-test, with a significance level of p < 0.05.