A crystalline germination stimulant (trivial name strigol) for the rootparasite, witchweed (Striga lutea Lour.), has been isolated from cotton rootexudates and characterized as a C(19)H(22)O(6) compound. Although apparently different from known plant hormones, the stimulant is active at hormonal levels, causing germination at concentrations less than 1O(-5) part per million.

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SllmmaryCD6 is a 130-kD glycoprotein expressed on the surface of thymocytes and peripheral blood T cells that is involved in TCK-mediated T cell activation. In thymus, CD6 mediates interactions between thymocytes and thymic epithelial (TE) cells. In indirect immunofluorescence assays, a recombinant CD6-immunoglobulin fusion protein (CD6-Kg) bound to cultured human TE cells and to thymic fibroblasts. CD6-Rg binding to TF and TE cells was trypsin sensitive, and 54 _+ 4% of binding was divalent cation dependent. By screening the blind panel of 479 monoclonal antibodies (mAbs) from the 5th International Workshop on Human Leukocyte Differentiation Antigens for expression on human TE cells and for the ability to block CD6-Rg binding to TE cells, we found one mAb 04-81) that significantly inhibited the binding of CD6-Kg to TE cells (60 _+ 7% inhibition). A second mAb to the surface antigen identified by mAb J4-81, J3-119, enhanced the binding of CD6-Kg to TE cells by 48 + 5%. Using covalent cross-linking and trypsin digestion, we found that mAb J4-81 and CD6-Kg both bound to the same 100-kD glycoprotein (CD6L100) on the surface of TE cells. These data demonstrate that a 100-kD glycoprotein on TE cells detected by mAb J4-81 is a ligand for CD6.

Cellular interactions between developing thymocytes and cells of the thymic microenvironment are necessary for maturation of thymocytes into mature T cells. While much is known about the molecules on developing T cells that mediate these interactions, little is known about the surface molecules of human thymic epithelial (TE) cells. In this study, using a panel of 276 MAb including 255 MAb from the 5th International Workshop on Human Leukocyte Differentiation Antigens (HLDA-V), we have determined the expression of CD1 through CDw130 and other surface molecules on resting and IFN-gamma-activated cultured human TE cells and on resting epidermal keratinocytes (EK). We demonstrate the surface expression of 50 of the 161 molecules assayed for on TE cells, including a number of adhesion molecules, cytokine receptors, Apo-1, and MHC-encoded molecules. While activation of TE cells with IFN-gamma for 48 hr induced a greater than fivefold increase in the expression of four surface molecules (CD38, CD54, MHC class I, and MHC class II), it also induced a greater than 50% increase in the expression of 14 other surface molecules (CD12, CD29, CD40, CD44, CD47, CD49b, CD49c, CD49e, CD55, CD66, CD87, CD104, TE4, and STE3) and a decrease in the expression of three molecules (CDw65, CDw109, and STE2). In comparing the phenotype of TE cells to 83 other cell lines studied in HLDA-V, we found that TE cells were strikingly more similar to EK than to any of the other cell types tested.

SllnunaryHuman thymic epithelial (TE) cells produce interleukin loe (Ibloe), I1:1~, and 11:6, cytokines that are important for thymocyte proliferation. The mRNAs for these cytokines are short-lived and are inducible by multiple stimuli. Thus, the steady-state levels for II.-1 and I1:6 mRNAs are critical in establishing the final cytokine protein levels. In this study we have evaluated the effect of epidermal growth factor (EGF), a growth factor for TE cells, and its homologue transforming growth factor oe (TGF-a), on primary cultures of normal human TE cells for the levels of I1:1oe, I1:1~, 1I.-6, and TGF-oe mRNA. We showed that TE cells expressed EGF receptors (EGF-R) in vitro and in vivo, and that treatment of TE cells with EGF or TGF-oe increased II.,1 and II.-6 biological activity and mRNA levels for I1:1oe, IL-lfl, and II-.6. Neither EGF nor TGF-c~ increased transcription rates of I1:1oe, II--18, and I1-.6 genes, but rather both EGF and TGF-ee increased cytokine mRNA stability. By indirect immunofluorescence assay, TGF-oe was localized in medullary TE cells and thymic Hassall's bodies while EGF-R was localized to TE cells throughout the thymus. Thus, TGF-oe and EGF are critical regulatory molecules for production of TE cell-derived cytokines within the thymus and may function as key modulators of human T cell development in vivo.

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