Identification of Initiator B Cells, a Novel Subset of Activation-Induced Deaminase-Dependent B-1-Like Cells That Mediate Initiation of Contact Sensitivity

Abstract

Contact sensitivity (CS) is related to delayed-type hypersensitivity and is a well-characterized prototype of T cell-mediated inflammation. However, the inflammatory response associated with CS is additionally dependent on Ag-specific IgM produced by a subpopulation of B cells in response to sensitization. Upon re-exposure to hapten, this IgM mediates rapid vascular activation and subsequent recruitment of proinflammatory T cells to the local site. Interference with this pathway prevents the full development of the classic delayed inflammatory response and is therefore termed the “CS initiation” pathway. In this study, we show that CS initiation is defective in mice deficient in activation-induced deaminase, an enzyme central to the process of somatic hypermutation. Using adoptive transfer experiments, we demonstrate that the defect is specific to a B-1-like population of B cells and that transfer of WT cells reconstitutes CS initiation mechanisms in deficient recipients. We went on to identify a novel subpopulation of Ag-binding B cells in the spleens of sensitized mice that possess initiation activity (CD19+CD5+Thy-1intIgMhighIgDhigh) that we name “initiator B cells.” Analysis of BCR H chain genes isolated from these cells revealed evidence of activation-induced deaminase-mediated somatic hypermutation. The sensitivity of CS initiation to very low amounts of sensitizing hapten suggests that the responsible B cells have increased IgM receptor gene mutations enabling selection to generate Abs with sufficient affinity to mediate the response.

Expression of activation‐induced cytidine deaminase enhances the clearance of pneumococcal pneumonia: evidence of a subpopulation of protective anti‐pneumococcal B1a cells

“…By analogy to the cutaneous hypersensitivities [CS and delayed-type hypersensitivity (DTH)], we postulated that B1a cells possibly were activated in the PerC within hours of infection and then migrated to the spleen within 2 days. [3][4][5]7,11 Accordingly, CD5 + CD19 + splenic B1a cells were purified from the spleens of WT donors with established pneumococcal infection. These B1a cells were then adoptively transferred i.v.…”

“…Note the similarity of this finding to our previous study of hapten-binding B1a cells that initiate CS, which were about 0Á05% of the total spleen cells, representing about 6000 cells per spleen. 4 In that system as few as 1500 cells transferred CS-initiation. 4 The AID-expressing B1a cell subpopulation shares some properties with memory B2 cells.…”

“…3,4 This subpopulation of B1a cells depends on the invariant natural killer T (iNKT) helper cytokine interleukin-4 (IL-4) for this effect. 4,5 In contrast conventional B1 cells are well known for producing NAb without somatic mutations and true antigen-specificity; instead they have low-affinity polyspecificity. This is due to germ-line V-region sequences and acquired random nucleotide (N) insertions Further, they are independent of T-cell helper cytokines.…”

“…FITCanti CD19 (1D3) and either phycoerythrin-anti CD5 (53-7.3) or phycoerythrin-anti CD11b monoclonal antibody; were sorted on a FACSVantage SE or a FACSAria flow cytometer (BD Biosciences), as described previously, 4 according to CD19 + CD11b + and CD19 + CD5 + phenotypes, respectively. Antibodies were purchased from BD Pharmingen (San Diego, CA).…”

“…4 Therefore we performed an in vivo CS-like anti-PC hapten experiment to verify this conceptual bridge. We infected mice as before and tested their cutaneous ear skin responses to the PC hapten associated with pneumococci in a protocol resembling those of the cutaneous hypersensitivities (CS and DTH).…”