B-1 cell

B-1 cells are B cells that express CD5, which can bind to another B cell surface protein, CD72. CD5-CD72 is thought to mediate B cell-B cell interaction. B-1 cells express IgM in greater quantities than IgG and its receptors show polyspecificity, meaning that they have low affinities for many different antigens, but have a preference for other immunoglobulins, self antigens and common bacterial polysaccharides. B-1 cells are present in low numbers in the lymph nodes and spleen and are instead found predominantly in the peritoneal and pleural cavities. B-1 cells generate diversity mainly via recombinatorial recombination (there is a preferential recombination between D-proximal VH gene segments). B-1 cells are first produced in the fetus (they then self renew in the periphery), unlike conventional B-2 cells that are produced after birth and replaced in the bone marrow.

B-1 B cells characteristically express high levels of sIgM, demonstrable CD11b, and low levels of sIgD, CD21, CD23, and the B cell isoform of CD45R (B220). In adult mice, B1 B cells constitute a minor fraction of the spleen and secondary lymphoid tissues but are enriched in the pleural and peritoneal cavities (1,2). B1 B cells were shown to arise from precursors in the fetal liver and neonatal but not adult bone marrow and constitute the earliest wave of mature peripheral B cells.

B1 B cells express a separable BCR repertoire (3). Sequence analysis indicates antibodies with restricted sets of V region genes, no evidence for somatic hypermutation (SHM), and few nontemplated nucleotide (N) sequence insertions, a pattern typical of neonatal B cells. Efficient B1 B cell development appears to be dependent on positive regulators of BCR signaling and the loss of negative regulators promotes greater accumulation of B1 B cells (4). Hence, there appears to be a role for self or foreign antigen in shaping the repertoire of the B1 B cell compartment (5).

B1 B cells self-renew and spontaneously secrete IgM and IgG3 serum antibodies. These natural serum antibodies display extensive polyreactivity (have low affinities for many different antigens, but have a preference for other immunoglobulins, self antigens and common bacterial polysaccharides) and demonstrable self-reactivity and bind to many common pathogen-associated carbohydrates (3). Natural serum antibodies play an important early role in the immune response to many bacteria and viruses but require complement fixation for effective antigen clearance. Innate sensing mechanisms can rapidly mobilize B1 B cells regardless of specificity highlighting the innatelike activity of this separate B cell compartment.

B1 B cells can be further subdivided into B1a (CD5+) and B1b (CD5-) subtypes. Unlike B1a B cells, the B1b subtype can be generated from precursors in the adult bone marrow. The B1a and B1b precursors have been reported to differ in the expression levels of CD138 (6). Recent functional studies indicate a further subdivision of labor assigning B1a cells as the precursors of natural serum antibody (7). In contrast, B1b cells appear to be the primary source of dynamic T cell independent (TI) antibody production and long-term protection after bacterial infection such as B. hermsii (8) and S. pneumonia (7). These studies indicate preexisting subset differences in BCR specificity and antigen-driven B cell fate that remain important unresolved features of the system.

Isolation of peritoneal B1 cells

In research laboratories, B1 B cells can be easily isolated from a mouse by injecting cell media or PBS into the peritoneal cavity of the mouse and then draining it off via a technique mirroring diagnostic peritoneal lavage. Cells can be identified and placed into 2 categories "B1a" or B1b" using multi-colour Flow cytometry looking for surface expression of CD19, B220, and CD5. B1a expresses high CD5 level, while B1b expresses low CD5 to almost-absent levels; both are CD19+ and B220+.

References

Berland R, Wortis HH. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Retrieve&list_uids=11861604 | Annu Rev Immunol. 2002;20:253-300. Epub 2001 Oct 4.]

1. Hayakawa K, Hardy RR, Herzenberg LA, et al. Progenitors for Ly-1 B cells are distinct from progenitors for other B cells. J Exp Med. 1985;161(6):1554–1568.

2. Lalor PA, Stall AM, Adams S, et al. Permanent alteration of the murine Ly-1 B repertoire due to selective depletion of Ly-1 B cells in neonatal animals. Eur J Immunol. 1989;19(3):501–506.

3. Kantor AB, Herzenberg LA. Origin of murine B cell lineages. Annu Rev Immunol. 1993;11:501–538.

4. Martin F, Kearney JF. B1 cells: similarities and differences with other B cell subsets. Curr Opin Immunol. 2001;13(2):195–201

5. Bendelac A, Bonneville M, Kearney JF. Autoreactivity by design: innate B and T lymphocytes. Nature Rev Immunol. 2001;1(3):177–186.

6. Tung JW, Mrazek MD, Yang Y, et al. Phenotypically distinct B cell development pathways map to the three B cell lineages in the mouse. Proc Natl Acad U S A. 2006;103(16):6293–6298.

7. Haas KM, Poe JC, Steeber DA, et al. B-1a and B-1b cells exhibit distinct developmental requirements and have unique functional roles in innate and adaptive immunity to S. pneumoniae. Immunity. 2005;23(1):7–18.

8. Alugupalli KR, Leong JM, Woodland RT, et al. B1b lymphocytes confer T cell-independent long-lasting immunity. Immunity. 2004;21(3):379–390

ee also

*B Cell
*CD5