Wekerle, Hartmut

Abstract BAFF and a proliferation-inducing ligand (APRIL), which control B cell homeostasis, are therapeutic targets in autoimmune diseases. TACI-Fc (atacicept), a soluble fusion protein containing the extracellular domain of the BAFF–APRIL receptor TACI, was applied in clinical trials. However, disease activity in multiple sclerosis unexpectedly increased, whereas in systemic lupus erythematosus, atacicept was beneficial. In this study, we show that an endogenous soluble TACI (sTACI) exists in vivo. TACI proteolysis involved shedding by a disintegrin and metalloproteinase 10 releasing sTACI from activated B cells. The membrane-bound stub was subsequently cleaved by γ-secretase reducing ligand-independent signaling of the remaining C-terminal fragment. The shed ectodomain assembled ligand independently in a homotypic way. It functioned as a decoy receptor inhibiting BAFF- and APRIL-mediated B cell survival and NF-κB activation. We determined sTACI levels in autoimmune diseases with established hyperactivation of the BAFF–APRIL system. sTACI levels were elevated both in the cerebrospinal fluid of the brain-restricted autoimmune disease multiple sclerosis correlating with intrathecal IgG production, as well as in the serum of the systemic autoimmune disease systemic lupus erythematosus correlating with disease activity. Together, we show that TACI is sequentially processed by a disintegrin and metalloproteinase 10 and γ-secretase. The released sTACI is an immunoregulator that shares decoy functions with atacicept. It reflects systemic and compartmentalized B cell accumulation and activation.

We describe a strategy to “revive” putatively pathogenic T cells from frozen specimens of human inflammatory target organs. To distinguish pathogenic from irrelevant bystander T cells, we focused on cells that were ( i ) clonally expanded and ( ii ) in direct morphological contact with a target cell. Using CDR3 spectratyping, we identified clonally expanded T cell receptor (TCR) β-chains in muscle sections of patients with inflammatory muscle diseases. By immunohistochemistry, we identified those Vβ-positive T cells that fulfilled the morphological criteria of myocytotoxicity and isolated them by laser microdissection. Next, we identified coexpressed pairs of TCR α- and β-chains by a multiplex PCR protocol, which allows the concomitant amplification of both chains from single cells. This concomitant amplification had not been achieved previously in histological sections, mainly because of the paucity of available anti-α-chain antibodies and the great heterogeneity of the α-chain genes. From muscle tissue of a patient with polymyositis, we isolated 64 T cells that expressed an expanded Vβ1 chain. In 23 of these cells, we identified the corresponding α-chain. Twenty of these 23 α-chains were identical, suggesting antigen-driven selection. After functional reconstitution of the αβ-pairs, their antigen-recognition properties could be studied. Our results open avenues for combined analysis of the full TCR α- and β-chain repertoire in human inflammatory tissues.