All ETDs from UAB

Advisory Committee Chair

Randall S Davis

Advisory Committee Members

Louise T Chow

Thomas M Ryan

Peter D Burrows

Louis B Justement

Document Type

Dissertation

Date of Award

2012

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Innate-like splenic marginal zone (MZ) and peritoneal cavity (PEC)-derived B1 B lymphocytes are the major contributors to primary humoral responses and play an important role in connecting innate and adaptive immunity, but are known to significantly differ in their B cell receptor (BCR) signaling characteristics. In mice, FCRL5 is discretely expressed by innate-like MZ and B1 B cells and is furthermore equipped with cytoplasmic ITAM-like and ITIM elements, suggesting diverse signaling potential for it. Our results showed that FCRL5 markedly inhibited BCR-mediated activation in MZ B cells, but not in PEC B1 B cells. To characterize its signaling function, a series of A20-IIA1.6 B cell line transductants engineered with chimeric receptors bearing cytoplasmic FCRL5 Y>F variants in frame with the extracellular portion of FcgRIIb were generated. Co-ligation of the WT FcgRIIb/FCRL5 chimeric protein with the BCR inhibited Ca2+ flux, whole-cell protein tyrosine phosphorylation, and Erk kinase activation via the ITIM-dependent (Y566) recruitment of the SHP-1 phosphatase. Conversely, co-ligation of a chimeric mutant bearing the intact ITAM-like (Y543/Y556) sequence with the BCR resulted in enhanced Ca2+ mobilization and Erk kinase activation. Intriguingly, this activation effect correlated with the Y543-dependent recruitment of the Lyn Src-family kinase (SFK). To further validate its bi-functional potential, viable SHP-1 deficient (mev/mev) and Lyn-/- mice were employed. These respective models revealed that both the SHP-1 phosphatase and Lyn kinase provide FCRL5 with counter-regulatory potential in modulating BCR signaling. Moreover, crosslinking FCRL5 with the BCR protected MZ B cells from BCR-mediated apoptosis, but promoted the death of B1 B cells. Finally, intracellular staining showed an excessive SHP-1 expression in MZ B cells that could confer FCRL5's inhibitory function in this particular subset. Taken together, these data indicate that FCRL5 has both inhibitory and activating signaling potential as well as a compartment-specific role in innate-like BCR signaling.

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