The TCR has been identified in several species as a heterodimer of two variable chains (reviewed in reference 1) that is associated with a set of invariant polypeptides collectively referred to as CD3 . MHC-restricted antigen recognition by both cytotoxic (mostly CD8') and helper (mostly CD4') T cells is mediated by clonotypic heterodimers of the a/ß type, while a second type of CD3-associated TCR, termed y/S, has recently been discovered on a small subpopulation of human (2) and mouse (3) T cells . mAbs to the TCR and CD3 molecules have been instrumental to the discovery and analysis of the TCR complex . Recent work in the mouse system has especially profited from the generation of mAbs to Vß segments expressed at a frequency detectable in unimmunized T cell populations (4-7), and to the invariant CD3E chain (8) . No mAb to . a constant determinant of the mouse TCR-a/ß is available, however, that could be used to discriminate TCR-a/ß and TCR-,y/8 expressing T cells and thymocytes. In the rat system, analysis of T cell maturation and activation has been hampered by a complete lack of TCR-and CD3-specific monoclonal reagents, despite an otherwise excellent collection of mAbs to cell surface molecules .Here, we describe a new mAb, termed R73, that detects a rat pan T cell surface antigen with the predicted properties of the TCR-a/ß on mature and immature cells of the T cell lineage and reports its functional effects on resting T lymphocytes . Materials and MethodsAnimals. Young adult Wistar and Lewis rats ofboth sexes were obtained from the animal quarters of the Max Planck Institute for Biochemistry, Martinsried, FRG, or from the Zentralinstitut für Versuchstierzucht, Medizinische Hochschule Hannover, FRG. Results obtained did not vary significantly between both strains .Immunization and Cell Fusion . Spleen cells from a BALB/c mouse alternately immunized intraperitoneally with rat T blasts and rat erythrocytes (it was also intended to generate an mAb to rat LFA-3) were fused 3 d after an intravenous injection of 10' rat erythrocytes with This work was funded by a grant from the Bundesministerium für Forschung und Technologie . Generation of the R73 cell line was funded by Genzentrum e.V. J . Exp. MED.

Activation of resting T lymphocytes is initiated by the interaction of cell-surface receptors with their corresponding ligands. In addition to activation through the CD3 (T3)-Ti antigen-receptor complex, recent experiments have demonstrated induction of T-cell proliferation through the CD2 (T11) molecule, traditionally known as the erythrocyte(E)-receptor, through which T cells can bind red blood cells (RBC). This 'alternative pathway' of T-cell activation was observed in vitro in response to combinations of anti-CD2 monoclonal antibodies (mAbs) that bind to distinct epitopes of CD2, such as mAbs against T11(2) plus T11(3). The physiological importance of this activation pathway can be assessed only by studying the effects of a naturally occurring ligand of CD2 on T-cell activation. We have recently described such a ligand, a glycoprotein of apparent relative molecular mass 42,000 (Mr 42K) that is expressed on all blood cells and some other tissues. Here we demonstrate that binding of this cell surface molecule, termed T11 target structure or T11TS, to CD2 (T11) induces reactivity in resting T cells to a mitogenic stimulus given by a mAb to the T11(3) determinant or by submitogenic concentrations of anti-T11(2+3) mAbs. Thus, one of the signals required for T-cell activation through the alternative pathway is provided by the interaction of CD2 with a naturally occurring complementary cell-surface molecule.

Expression of the rat CD8 molecule was studied using five novel monoclonal antibodies (mAb), four of which are specific for the V-like domain of CD8 alpha, whereas one reacts either with the beta chain or with a determinant only expressed on the CD8 alpha/beta heterodimer. mAb to both chains effectively blocked purified lymph node CD8 T cells in mixed lymphocyte reaction and in cell-mediated cytotoxicity. Flow cytometric analysis showed that CD8 T cells from lymph nodes or spleen of normal rats almost exclusively express the alpha/beta isoform, regardless of the T cell receptor isotype (alpha/beta or gamma/delta). In contrast, natural killer (NK) cells carry only CD8 alpha chains. This CD8 alpha + beta - phenotype was also prominent among CD8 T cells from athymic rats and from intestinal epithelium of normal rats. CD8 alpha homodimers can also be expressed as a result of activation, as shown by analysis of CD4 CD8 double-positive T cells obtained from highly purified lymph node CD4 T cells by in vitrok stimulation. Such CD4+CD8 alpha + beta - cells also represent a major subset among adult intestinal intraepithelial lymphocytes (IEL), suggesting local activation. Taken together, the difference in CD8 isoform expression among T cells from athymic rats, NK cells, and gut IEL versus CD8 T cells from peripheral lymphatic organs of euthymic animals suggests that like in mice, expression of the CD8 heterodimer is more dependent on intrathymic maturation than that of the homodimer. Since the more stringent thymus dependence of CD8 alpha + beta + T cells may be due to a requirement for thymic selection on self major histocompatibility complex class I antigens, the virtually exclusive CD8 alpha + beta + phenotype of peripheral rat gamma/delta T cells could mean that antigen recognition by this subset is also restricted by MHC class I molecules.

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Necrotizing enterocolitis (NEC) is a severe, currently untreatable intestinal disease that predominantly affects preterm infants and is driven by poorly characterized inflammatory pathways. Here, human and murine NEC intestines exhibit an unexpected predominance of type 3/TH17 polarization. In murine NEC, pro-inflammatory type 3 NKp46−RORγt+Tbet+ innate lymphoid cells (ILC3) are 5-fold increased, whereas ILC1 and protective NKp46+RORγt+ ILC3 are obliterated. Both species exhibit dysregulation of intestinal TLR repertoires, with TLR4 and TLR8 increased, but TLR5-7 and TLR9-12 reduced. Transgenic IL-37 effectively protects mice from intestinal injury and mortality, whilst exogenous IL-37 is only modestly efficacious. Mechanistically, IL-37 favorably modulates immune homeostasis, TLR repertoires and microbial diversity. Moreover, IL-37 and its receptor IL-1R8 are reduced in human NEC epithelia, and IL-37 is lower in blood monocytes from infants with NEC and/or lower birthweight. Our results on NEC pathomechanisms thus implicate type 3 cytokines, TLRs and IL-37 as potential targets for novel NEC therapies.