Human memory B cells isolated from blood and tonsils are functionally distinctive

Abstract

Human tonsils are lymphoepithelial structures considered comparable to the nasopharynx associated lymphoid tissue (NALT) in rodents, component part of the mucosa-associated lymphoid tissue (MALT). Nevertheless, regarding NALT, humans and mice are histologically and anatomically divergent, being studies based on rodents doubtfully representative of the situation in humans. Tonsils establish direct interactions with inhaled or ingested environmental antigens (1). They show similarities with lymph nodes and could participate as effector organs of local systemic type as well as mucosal secretory type of adaptive humoral immunity. There, B cell (Bc) encounter with antigen (Ag), followed by cognate T-cell help, drives proliferation of Ag-specific naive Bc and their differentiation into memory Bc (Bmem) and plasma cells. Bmem can be in turn re-stimulated, expanded, selected, and turned into effector cells. Bmem reside within discrete regions of secondary lymphoid tissue or reenter and become detectable in the bloodstream (2). Human Bc studies in vitro, routinely have used Bc purified from spleen, blood or tonsils irrespective of potential differences in their immunological traits. In this dissertation we will present our recent findings (3) regarding differences in the functional response in vitro, of tonsillar and peripheral total Bc as well as sorted Bmem. Our observations clearly reveal that a major proportion of tonsillar Bmem differentiated rapidly but exhibited little expansion and greater cell death, compatible with their higher initial activation status. In contrast, circulating Bmem were slower to commence proliferation and differentiation, but showed much higher survival rates. We have used a number of immunological methods, combining FACS, cell sorting, Luminex technology and ELISA. We particularly put considerable effort into working under physiological conditions in vitro using always freshly isolated human B cells from both tissues and cultured them simultaneously. Our results hold relevant implications for the B cell field. In principle, they represent novel data supporting a conventional notion: cells become more and more terminally differentiated with each successive antigenic stimulus and cell division (4). Tonsils are easily reachable to loads of antigens, and we show tonsillar Bmem tendency to cell death and their decreased potential to proliferate in comparison with their peripheral counterparts, usually patrolling sterile tissues. These led us to propose that NALT Bmem are adapted to a strong and immediate response on site that is not sustained in time, whilst long-term maintenance of Bmem would be dependent on cells that retain proliferative capacity, such as circulating Bmem. Further inferences in relation to vaccination and future perspectives will be discussed.