BAFF-R, a newly identified TNF receptor that specifically interacts with BAFF.

B cell homeostasis has been shown to critically depend on BAFF, the B cell activation factor from the tumor necrosis factor (TNF) family. Although BAFF is already known to bind two receptors, BCMA and TACI, we have identified a third receptor for BAFF that we have termed BAFF-R. BAFF-R binding appears to be highly specific for BAFF, suggesting a unique role for this ligand-receptor interaction. Consistent with this, the BAFF-R locus is disrupted in A/WySnJ mice, which display a B cell phenotype qualitatively similar to that of the BAFF-deficient mice. Thus, BAFF-R appears to be the principal receptor for BAFF-mediated mature B cell survival.

Molecular mapping with functional antibodies localizes critical sites on the human IL receptor common gamma (gamma c) chain.

The IL receptor common gamma (gamma c) chain is required for the formation of high affinity cytokine receptor complexes for IL-2, IL-4, IL-7, IL-9, and IL-15, and for signals regulating cell survival, growth, and differentiation. Our current understanding of how gamma c chain associates with multiple ligands and receptor subunits is drawn largely from its structural homology to the human growth hormone (hGH) receptor and known structure of the hGH/hGH receptor complex. These receptors share distinct features in their extracellular portions and are believed to function by a mechanism of ligand-induced association of receptor subunits. Here, we report the first directed mutational analysis of the human gamma c chain by alanine scanning conducted across seven regions likely to contain residues required for intermolecular contact. Functionally distinct, neutralizing anti-gamma c mAbs were employed to define critical residues. One particular mAb, CP.B8, unique in its ability to inhibit IL-2-, IL-4-, IL-7-, and IL-15-induced proliferation and high affinity cytokine binding of normal T cells as an intact mAb and as a Fab fragment, localized critical residues to four noncontinuous stretches, namely residues in loops AB and EF of domain 1, in the interdomain segment, and in loop FG of domain 2. Notably, these residues form a contiguous patch on the gamma c chain surface in a three-dimensional structural model. These results provide functional evidence for the location of contact points on gamma c chain required for its association with multiple ligands.

Antibodies to lymphotoxin alpha (LT alpha) and LT beta recognize different glial cell types in the central nervous system.

The cytokine lymphotoxin (LT) is known to exist in two forms, secreted LT alpha and a membrane-bound LT alpha/beta complex. LT alpha shares the same receptor as tumor necrosis factor alpha and LT beta is recognized by its receptor, LT betaR. Since LT has been associated with oligodendrocyte pathology, the present study has examined the expression of these molecules by immunocytochemistry in diseased and normal CNS tissue, with a panel of monoclonal antibodies (mAb) to LT alpha, LT beta and LT betaR. Of three mAb to LT beta, two (B27 and C37) gave specific membrane staining on astrocytes, as well as lymphocytes. The third anti-LT beta mAb, B9, was selectively immunoreactive for oligodendrocytes, suggesting specific recognition sites. The reactivity was not specific for multiple sclerosis (MS) since oligodendrocytes in normal and non-MS CNS tissue also displayed positivity. MAb to LT betaR reacted with astrocytes only, giving a punctate membrane staining pattern suggestive of receptor sites. MAb to LT alpha gave strong reactivity on lymphocytes in active MS lesions and weak reactivity on microglia within lesion areas. These results show that mAb to LT alpha and LT beta recognize different cell types within the CNS. Furthermore, individual mAb against LT beta were capable of distinguishing between astrocytes and oligodendrocytes, perhaps indicative of different epitopes on LT beta. The presence of LT betaR on astrocytes suggests possible interactions between infiltrating lymphocytes and astrocytes via the LT pathway.

Characterization of lymphotoxin-alpha beta complexes on the surface of mouse lymphocytes.

The lymphotoxin-alpha beta complex (LT alpha beta) is found on the surface of activated lymphocytes and binds to a specific receptor called the LT beta receptor (LT beta R). In the mouse, signaling through this pathway is important for lymph node development and splenic organization, yet the biochemical properties of murine LT alpha and LT beta are essentially unknown. Here we have used soluble receptor-Ig forms of LT beta R and TNF-R55 and mAbs specific for murine LT alpha, LT beta, and LT beta R to characterize the appearance of surface LT alpha beta complexes and LT beta R on several common murine cell lines. Cells that bound LT beta R also bound anti-LT alpha and anti-LT beta mAbs in a FACS analysis. The ability of these reagents to discriminate between surface TNF and LT was verified by analysis of surface TNF-positive, LPS-activated murine RAW 264.7 monocytic cells. Primary mouse leukocytes from spleen, thymus, lymph node, and peritoneum were activated in vitro, and CD4+ and CD8+ T cells as well as B cells expressed surface LT ligand but not the LT beta R. Conversely, elicited peritoneal monocytes/macrophages were surface LT negative yet LT beta R positive. This study shows that on mononuclear cells, surface LT complexes and receptor are expressed similarly in mice and man, and the tools described herein form the foundation for study of the functional roles of the LT system in the mouse.

Cytotoxic activities of recombinant soluble murine lymphotoxin-alpha and lymphotoxin-alpha beta complexes.

Human lymphotoxin-alpha (LT alpha) is found in a secreted form and on the surface of lymphocytes as a complex with a second related protein called lymphotoxin-beta (LT beta). Both secreted human LT alpha and TNF have similar biological activities mediated via the TNF receptors, whereas the cell surface LT alpha beta complex binds to a separate receptor called the LT beta receptor (LT beta R). The murine LT alpha and LT beta (mLT alpha and mLT beta) proteins have never been characterized. When recombinant mLT alpha was produced by either of several methods, the protein had a very low specific activity relative to that of human LT alpha in the conventional WEHI 164 cytotoxicity bioassay. The weak activity observed was inhibited by a soluble murine TNF-R55 Ig fusion protein (mTNF-R55-Ig), but not by mLT beta R-Ig. Coexpression of both mLT alpha and a soluble version of mLT beta in insect cells led to an LT alpha beta form that was cytotoxic in the WEHI 164 assay via the LT beta R. To determine whether natural mLT alpha-like forms with cytotoxic activity comparable to that of secreted human LT alpha were secreted from primary spleen cells, splenic lymphocytes were activated in various ways, and their supernatants were analyzed for cytotoxic activity. Using specific Abs to distinguish between mTNF and mLT, a TNF component was readily detected; however, there was no evidence for a secreted mLT alpha cytotoxic activity using this assay. Combined, these observations suggest that secreted mLT alpha may not play a role in the mouse via interactions with TNF-R55, and the ramifications of this hypothesis are discussed.