Natural antibodies (NAbs) are most commonly defined as immunoglobulins present in the absence of exogenous antigen stimulation. In fact, numerous groups have demonstrated the presence of NAbs in both specific pathogen-free and germ-free mice (1–3). These NAbs provide immediate protection against infection while the adaptive arm of the immune system mounts a specific and long-lasting response. Beyond immediate protection from infection, NAbs have been shown to play various functional roles in the immune system, which include clearance of apoptotic debris (Gronwall et al.), suppression of allergic responses (4, 5), regulation of B cell responses (6), selection of the B cell repertoire (7, 8), protection from cancer (9, 10), regulation of B cell development [Baumgarth;(7, 11)], and protection against atherosclerosis (12–15). These various functions of NAbs are afforded by their reactivity, which is broad, cross-reactive, and shown to recognize evolutionarily fixed epitopes present in foreign antigens [Gronwall et al.;(16–21)]. Furthermore, NAbs have unique characteristics that also contribute to their functional roles and set them apart from antigen-specific antibodies. Such characteristics include germline structure (lacking non-templated nucleotides and little to no somatic hypermutation) and a restricted repertoire (16, 22–24).

Determining and subsequently examining the B cells producing NAbs have been the subject of intense investigation since the early 1980s despite NAbs being studied since the late 1960s. NAb producing B-1a cells were first identified in mice and characterized by surface expression of CD5+, IgM high, IgD low, CD19 high, B220 low, CD23−, and CD43+(25), which contrasts with the surface phenotype of follicular B-2 cells: CD5−, IgM low, IgD high, CD19+, B220+, CD23+, and CD43−. Studies have demonstrated that B-1a cells are found in the peritoneal cavity, pleural cavity, spleen, bone marrow, lymph nodes, and blood of mice (26). Furthermore, various subsets of B-1a cells have been identified and include those expressing PD-L2 (PD-L2+/−)(27, 28), CD25 (CD25+/−)(Tumang et al.), CD73 (CD73 hi/lo)(29), and PC-1 (PC-1 hi/lo). Throughout the many years of B-1a cell investigation, it has been shown that not all subsets of murine B-1a cells secrete NAbs. This has important implications when investigating the source of protective and/or pathogenic NAbs.