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Summary: B cell activating factor (BAFF) provides B cells with essential survival signals. It binds to three receptors: BAFFR, TACI, and BCMA that are differentially expressed by B cell subsets. BAFFR is early expressed in circulating B cells and provides key signals for further maturation. Here, we report that highly regulated BAFFR processing events modulate BAFF responses. BAFFR processing is triggered by BAFF binding in B cells co-expressing TACI and it is executed by the metalloproteases ADAM10 and ADAM17. The degree of BAFF oligomerization, the expression of ADAM proteins in different B cell subsets, and the activation status of the cell determine the proteases involved in BAFFR processing. Inhibition of ADAM10 augments BAFF-dependent survival of primary human B cells, whereas inhibition of ADAM17 increases BAFFR expression levels on germinal center B cells. Therefore, BAFF-induced processing of BAFFR regulates BAFF-mediated B cell responses in a TACI-dependent manner. : Smulski et al. report that the B cell survival receptor BAFFR undergoes ligand-induced shedding but only in cells co-expressing a second receptor for BAFF called TACI. BAFFR shedding can be performed by ADAM10 in circulating B cells or by ADAM17 in germinal center B cells and limits BAFF-mediated survival signals. Keywords: B cell, BAFF-receptor, BAFFR, TACI, ADAM10, ADAM17, metalloprotease, processing, ectodomain shedding, germinal center, survival

A 78 kDa protein was induced in cat peripheral blood mononuclear cells after in vivo administration of recombinant human interferon-alpha hybrid (rHuIFN-alpha B/D). This protein was antigenically related to the IFN-induced human (78 kDa) and mouse Mx proteins. Quantitative immunoblot analysis indicated that the induction of the cat Mx protein was dose-dependent. There was a dissociation in time between plasma levels of IFN which were transient, and levels of cat Mx protein which remained elevated at least five days after dosing. Our results provide evidence that a human IFN-alpha hybrid may be active in cats. They also indicate that the Mx protein is a sensitive, quantitative, and stable marker to follow IFN activity in vivo in cats.