Much has been learned about the structure, function, and production of IgM, since the antibody's initial characterization. It is widely accepted that IgM provides a first line of defense during microbial infections, prior to the generation of adaptive, high affinity IgG responses that are important for longlived immunity and immunological memory. Although IgM responses are commonly used as a measure of exposure to infectious diseases, it is perhaps surprising that the role of and requirement for IgM in many microbial infections has not been well explored in vivo. This is in part due to the lack of capabilities, until relatively recently, to evaluate the requirement for IgM in the absence of coincident IgG responses. Such evaluations are now possible, using gene-targeted mouse strains that produce only IgM, or isotype-switched IgG. A number of studies have revealed that IgM, produced either innately, or in response to antigen challenge, plays an important and perhaps underappreciated role in many microbial infections. Moreover, the characterization of the roles of various B cell subsets, in the production of IgM, and in host defense, has revealed important and divergent roles for B-1a and B-1b cells. This review will highlight studies in which IgM, in its own right, has been found to play an important role, not only in early immunity, but also in long-term protection, against a variety of microbial pathogens. Observations that long-lived IgM responses can be generated in vivo suggest that it may be feasible to target IgM production as part of vaccination strategies. KeywordsIgM; microbial infection; B-1 cell Properties of IgM important for microbial immunityThe characteristics and functions of IgM have been reviewed widely, and have been well documented (for review, see [1]). Many properties of IgM make this immunoglobulin particularly well-suited to its role in microbial immunity. It is present in high concentrations in blood (in the range of 1.5mg/ml), and is the first antibody elicited in an immune response following immunization or infection. IgM has a relatively short half-life in the serum, approximately 28 hours, in normal mice in the absence of antigen [2]. Thus, it is generally assumed that the production of IgM wanes, once B cell responses mature. However, some recent evidence, including our own, suggests that this is not always be the case, and 1 Address correspondence and reprint requests to Gary

Although T-independent immunity is known to be generated against bacterial capsular and cell wall polysaccharides expressed by a number of bacterial pathogens, it has not been studied in depth during intracellular bacterial infections. Our previous study demonstrated that Ehrlichia muris, an obligate intracellular tick-borne pathogen, generates protective classical TI responses in CD4 T cell-deficient C57BL/6 mice. We found that E. muris T-independent immunity is accompanied by the expansion of a very large extrafollicular spleen population of CD11clow-expressing plasmablasts that exhibit characteristics of both B-1 and marginal zone B cells. The plasmablasts comprised up to 15% of the total spleen lymphocytes and ∼70% of total spleen IgMhighIgDlow cells during peak infection in both wild-type and MHC class II-deficient mice. The CD11clow cells exhibited low surface expression of B220, CD19, and CD1d, high expression of CD11b, CD43, but did not express CD5. Approximately 50% of the CD11clow cells also expressed CD138. In addition to CD11b and CD11c, the plasmablasts expressed the β1 (CD29) and α4 (CD49d) integrins, as well as the chemokine receptor CXCR4, molecules which may play roles in localizing the B cells extrafollicular region of the spleen. During peak infection, the CD11clow cells accounted for the majority of the IgM-producing splenic B cells and nearly all of the E. muris outer membrane protein-specific IgM-secreting cells. Thus, during this intracellular bacterial infection, CD11c expression identifies a population of Ag-specific spleen plasmablasts responsible for T-independent Ab production.

Immunological memory has long considered to be harbored in B cells that express high affinity class-switched IgG. IgM-positive memory B cells can also be generated following immunization, although their physiological role has been unclear. Here we show that bacterial infection elicited a relatively large population of IgM memory B cells that were uniquely identified by their surface expression of CD11c, CD73, and PD-L2. The cells lacked expression of cell surface markers typically expressed by GC B cells, were CD138-negative, and did not secrete antibody ex vivo. The population was also largely quiescent, and accumulated somatic mutations. The IgM memory B cells were located in the region of the splenic marginal zone, and were not detected in blood or other secondary lymphoid organs. Generation of the memory cells was CD4 T cell-dependent, and required IL-21R signaling. In vivo depletion of the IgM memory B cells abrogated the IgG recall responses to specific antigen challenge, demonstrating that the cell population was required for humoral memory, and underwent class switch recombination following antigen encounter. Our findings demonstrate that T cell-dependent IgM memory B cells can be elicited at high frequency, and can play an important role in maintaining long-term immunity during bacterial infection.