Anthony Defranco, PhD

The DeFranco lab studies signal transduction by the B cell antigen receptor (BCR) and by Toll-like receptors (TLRs) and is particularly interested in how signaling supports the proper functioning of the immune system.  They are also interested in how alterations in these signaling pathways may lead to disease.

BCR signaling involves activation of intracellular protein tyrosine kinases of three types and downstream activation of a variety of signaling reactions.  Although Src-family tyrosine kinases play a very important role in the initiation of signaling, B cells express three family members and one of these, Lyn, also is important for feedback inhibition of BCR signaling.  In the absence of Lyn, BCR signaling proceeds with a slight delay, but feedback inhibition is defective, resulting in exaggerated signaling and spontaneous autoimmune disease that resembles the human disease systemic lupus erythrematosus.  A major project in the lab involves studying how deficiency in Lyn leads to defects in B cell tolerance and spontaneous autoimmune disease.  These studies are part of a long-term collaboration with Clifford Lowell, who studies the function of Lyn in myeloid cells and how alterations in myeloid cells also contribute to the autoimmunity observed in the Lyn-/- mice.  A second major project in this area is to understand how BCR signaling is differentially regulated between immature and mature B cells.  Immature B cells are highly sensitive to antigen, leading to tolerance-related responses such as receptor editing and anergy, whereas mature B cells are less sensitive but exhibit robust signaling when the BCR is strongly engaged, a dose response behavior which likely reflects a threshold for activation.  One of the feedback inhibitory pathways controlled by Lyn (involving CD22) is strongly active in mature B cells and less active in immature B cells, but this does not account for all differences in the sensitivity to antigen.  Recently we have begun to analyze how the diacylglycerol kinases may contribute to regulation of the activation of mature B cells.

TLR signaling is important for many immune responses and occurs in most if not all immune cell types in response to infections.  For this reason, it has been hard to distinguish the relative contributions of different cell types for TLR-dependent immune reactions.  To address this problem, we have created a conditional allele of the critical TLR signaling component Myd88 in which an essential exon is surrounded by loxP sites.  Cell type-specific expression of the Cre recombinase results in deletion of the Myd88 gene in that cell type.  We have used mice in which Myd88 is deleted to demonstrate that TLR signaling in dendritic cells plays an essential role for the innate immune restriction of growth by the single-celled parasite Toxoplasma gondii.  Similarly, Myd88 in dendritic cells was required for a normal magnitude IgG response to soluble protein antigens covalently coupled to a TLR ligand.  In contrast, Myd88 function in B cells, but not dendritic cells was found to greatly enhance the IgG germinal center response to virus particles.  This may represent an adaptation of the immune system designed to boost antibody responses to virus particles. These studies illustrate that TLR signaling is important for many immune responses and begin to define how this occurs in vivo.