Anthony L. DeFranco
University of California, San Francisco
The B cell antigen receptor (BCR) regulates B cell development, tolerance to self, activation, and continued survival. Antigen-induced oligomerization of the BCR leads to phosphorylation by the Src-family tyrosine kinases Lyn, Fyn, and Btk of key tyrosines within the Ig-
a
and Ig-
b
subunits of the BCR in a conserved sequence called the ITAM for
Immunoreceptor Tyrosine-based Activation Motif
The phosphorylated ITAM serves as a binding site for the cytosolic tyrosine kinase, Syk. Downstream signaling events are thought to be largely dependent upon Syk, although Src-family members may also participate. Among the Src-family members, Lyn is the most abundant, accounting for approximately two-thirds of the total activity and protein in B cells. We have examined mice in which the Lyn gene has been ablated by gene targeting. B cells from young adult Lyn-deficient animals are fairly normal, although they contain a higher proportion of immature B cells than do B cells from wild type mice and more cells that are short-lived. The initial BCR signaling events were delayed somewhat, consistent with the idea that the Src-family members play an important role in initiating BCR signaling events. Although the BCR-induced calcium increase was delayed in Lyn-deficient B cells, the peak and sustained levels of calcium were greater than wild type after the initial period. Activation of the Erk1/Erk2 MAP kinases was also greater in Lyn-deficient B cells and these B cells proliferated to a greater degree than their wild type counterparts. These results suggest Lyn has two types of roles in BCR signaling, a positive role in the initiation of signaling and a negative role with regard to at least some signaling reactions. Consistent with this interpretation, the inhibitory function of Fc
g
RIIb1 was partially defective in Lyn-deficient B cells, and the ability of CD22 to inhibit calcium elevation was absent. The inhibitory functions of Fc
g
RIIb1 and CD22 depend upon phosphorylation of tyrosines in their cytoplasmic domains and subsequent recruitment of the signaling inhibitors SHIP and SHP-1 to them. Tyrosine phosphorylation of the cytoplasmic tail of the FcR was substantially reduced, and tyrosine phosphorylation of CD22 was absent in Lyn-deficient B cells, suggesting that Lyn is the primary kinase that phosphorylates these negative regulators. As Lyn-deficient mice age, they exhibit a large expansion in B lymphoblastoid cells with a B-1 phenotype and IgM-secreting plasma cells. They produce substantial amounts of auto-reactive antibodies and deposits of immune complexes in their kidneys, indicative of autoimmune disease. These phenomena may reflect B cell hyper-reactivity resulting from the loss of inhibitory influences to BCR signaling contributed by Lyn.
