Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Michael C. Rahe, Michael P. Murtaugh, Gourapura J. Renukaradhya.
Immunological prevention of infectious disease, especially viral, is based on antigen-specific long-lived memory B cells. To test for cellular proliferation and differentiation factors in swine, an outbred model for humans, CD21+ B cells were activated in vitro with CD40L and stimulated with purported stimulatory cytokines to characterize functional responses. IL-21 induced a 3-fold expansion in total cell numbers with roughly 15% of all B cells differentiating to IgM or IgG antibody secreting cells (ASCs.) However, even with robust proliferation, cellular viability rapidly deteriorated. Therefore, a proliferation inducing ligand (APRIL) and B cell activating factor (BAFF) were evaluated as survival and maintenance factors. BAFF was effective at enhancing the viability of mature B cells as well as ASCs, while APRIL was only effective for ASCs. Both cytokines increased approximately two-fold the amount of IgM and IgG which was secreted by IL-21 differentiated ASCs. Mature B cells from porcine reproductive and respiratory virus (PRRSV) immune and naïve age-matched pigs were activated and treated with IL-21 and then tested for memory cell differentiation using a PRRSV non-structural protein 7 ELISPOT and ELISA. PRRSV immune pigs were positive on both ELISPOT and ELISA while naïve animals were negative on both assays. These results highlight the IL-21-driven expansion and differentiation of memory B cells in vitro without stimulation of the surface immunoglobulin receptor complex, as well as the establishment of a defined memory B cell culture system for characterization of vaccine responses in outbred animals.
The memory B cell is a critical component of protective long-term immunity against reinfection. Following antigenic recognition, its ability to rapidly proliferate and differentiate into antibody secreting cells (ASC) results in the production of antigen-specific antibodies. These antibodies are essential for binding and clearance of invading pathogens prior to the incidence of clinical disease. Previous in vitro work in the pig has shown that this secondary humoral immune response requires antigen specific T cell help [1, 2]. However, the factors necessary to stimulate robust porcine B cell expansion and differentiation to ASCs have not been extensively studied, except in a mixed leukocyte culture system [3, 4]. Work on human and mouse B cells has shown that, while many cytokines are capable of producing a proliferative and differentiating response, IL-21 is the most potent at driving this response .
IL-21 plays a critical role in the adaptive immune response due to its ability to stimulate B cell differentiation and antibody production, and to promote the maladaptive development of inflammatory disease and autoimmune disorders, such as type 1 diabetes [39, 40]. However, limited data on IL-21 activities in the pig hinders progress in development of the pig for organ and islet xenotransplantation. It is of particular importance for type 1 diabetes, as IL-21 production is increased in type 1 diabetic patients, and resulting B cell involvement in disease progression seems likely [41, 42]. Furthermore, the cross-species activities of human CD40L and IL-21 on porcine B cells highlights the unanticipated host-graft interactions that may complicate xenotransplant acceptance.