Double-Positive CD21+CD27+B Cells Are Highly Proliferating Memory Cells and Their Distribution Differs in Mucosal and Peripheral Tissues

Background: Several B-cell defects arise in HIV infected patients, particularly in patients with chronic infection and high viral load. Loss of memory B cells (CD27(+) B cells) in peripheral blood and lymphoid tissues is one of the major B cell dysfunctions in HIV and simian immunodeficiency virus (SIV) infection. Despite several studies, definitive identification of memory B cells based on CD27 surface expression has not been described. Similarly, the rates of cell turnover in different B cell subpopulation from lymphoid and mucosal tissues have not been well documented. In this study, we demonstrate the presence of memory B cell populations and define their distribution, frequency and immunophenotype with regards to activation, proliferation, maturation, and antibody production in normal rhesus macaques from different lymphoid tissues. Methodology/Principal Findings: Thirteen healthy, uninfected rhesus macaques were selected for this study. CD20(+) B cells were isolated from peripheral blood and sorted based on CD27 and CD21 surface markers to define memory B cell population. All the B cell subpopulation was further characterized phenotypically and their cell turnover rates were evaluated in vivo following bromodeoxyuridine (BrdU) inoculation. Double positive (DP) CD21(+)CD27(+) B cells in both peripheral and lymphoid tissues are memory B cells, able to produce antibody by polyclonal activation, and without T cell help. Peripheral and lymphoid DP CD21(+)CD27(+) B cells were also able to become activated and proliferate at higher rates than other B cell subpopulations. Increased turnover of tonsillar memory B cells were identified compared to other tissues examined. Conclusions/Significance: We suggest that this DP memory B cells play a major role in the immune system and their function and proliferation might have an important role in HIV/SIV mediated B cell dysregulation and pathogenesis.