B1‐ and CD5‐Positive B Cells


B1 (B1a/B1b) cells represent a population distinct from conventional B2 cells, which predominates the coelomic cavities. In contrast to B2 cells, B1 cells are selected by self‐antigens, and are chronically active in an autonomous B cell receptor pathway. B1 cells produce circulating IgM natural antibodies that provide ‘innate‐like’ protection against pathogens, contribute to the elimination of apoptotic cells and prevent B1 cell expansion. Furthermore, B1 cells are effective both in presenting autoantigens to T cells, and in inducing T cell activation and differentiation. The vast majority of B1 cells express CD5 and the new findings suggest that part of the B1 cell properties, as well as the capacity of B1 cells to contribute to the development of autoimmune diseases and chronic lymphocytic leukaemia, is related to the expression of the CD5 molecule.

Key Concepts:

  • B1 cells are a subclass of B cells.

  • B1 cells are involved in the innate protection.

  • B1 cells are first produced in the foetus.

  • B1 cells predominate in the coelomic cavities in adults.

  • B1 cells overlap incompletely with CD5+ B cells in humans.

Keywords: B1; B2; CD5; natural antibody; tolerance

Figure 1.

B1, B2 and regulatory B cell subsets phenotype. B1 cells can be distinguished from B2 cells on the basis of CD19high, CD23, CD43+, IgMhigh, IgDvariable and CD5 expression (CD5+ in B1a, and CD5 in B1b). Follicular B cells and marginal zone B cells, which form the B2 subset, differ from each other in terms of phenotype and localisation. The regulatory B cell subset has phenotypical markers of both B1 and B2 cells.

Figure 2.

Schematic representation of the CD5 molecule. Abbreviations: aa, amino acids; CP, connecting peptide; CY, cytoplasmic tail; D1, first extracellular domain; D2, second extracellular domain; D3, third extracellular domain; ITAM, immunoreceptor tyrosine‐based activation‐like motif (consensus YxxI/Lx6–12YxxI/L); ITIM, immunoreceptor tyrosine‐based inhibition like motif (consensus S/I/V/LxYxxI/V/L); L, leader peptide; TM, transmembrane region. Intracellular signalling molecules that associate with the CD5 cytoplasmic tail are SHP‐1, RasGap, C‐cbl (negative regulators), lck, CaMKII, PI3K, PKC/G (activation molecules) and CK2 (survival factor).

Figure 3.

cd6 and cd5 genes map to contiguous regions and are supposed to have arisen from duplication of a common ancestral gene probably before the separation of bird/mammalian and fish/amphibians 200–300 Ma. The enhancer and microsatellite sequences of cd5 gene are indicated. In humans, not in mice, an endogenous retrovirus type E (HERV‐CD5) is inserted between cd6 and cd5 genes, and serves as an alternative promoter for CD5.



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Further Reading

Baumgarth J and Poser R (2010) A rare cause for supraclavicular lymph node metastases. Laryngo‐Rhino‐Otologie 89(12): 754–755.

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Renaudineau, Yves, Bariller, Edwige, and Olivier Pers, Jacques(Feb 2014) B1‐ and CD5‐Positive B Cells. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0024242]