Zhu, L.Y., Lin, A.F., Shao, T., Nie, L., Dong, W.R., Xiang, L.X., and Shao, J.Z. (2014) B cells in teleost fish act as pivotal initiating APCs in priming adaptive immunity: an evolutionary perspective on the origin of the B-1 cell subset and B7 molecules. Journal of immunology (Baltimore, Md. : 1950). 192(6):2699-2714.
The long-held paradigm that B cells cannot uptake nonspecific particulate Ags for the initiation of primary adaptive immunity
has been challenged by the recent discovery that teleost B cells have potent phagocytic and microbicidal abilities. This discovery
provides preliminary clues that primitive B cells might act as initiating APCs in priming adaptive immunity. In this study,
zebrafish B cells clearly showed a potent Ag-presenting ability to both soluble Ags and bacterial particles to prime naive
CD4+ T cell activation. This finding demonstrates the innate-like nature of teleost B cells in the interface of innate and adaptive
immunity, indicating that they might consist of a major population of initiating APCs whose performance is similar to that
of dendritic cells. Given the functional similarities between teleost B cells and the mammalian B-1 subset, we hypothesize
that B-1 lineage and teleost B cells might originate from a common ancestor with potent phagocytic and initiating APC capacities.
In addition, CD80/86 and CD83 costimulatory signals were identified as being essential for B cell–initiated adaptive immunity.
This result suggests that the costimulatory mechanism originated as early as the origin of adaptive immunity and is conserved
throughout vertebrate evolution. In fish, only a single CD80/86 copy exists, which is similar to mammalian CD86 rather than
to CD80. Thus, CD86 might be a more primordial B7 family member that originated from fish. This study provides valuable insights
into the evolutionary history of professional APCs, B cell lineages, and the costimulatory mechanism underlying adaptive immunity
as a whole.