Vitamin A and its biologically active derivatives, the retinoids, are recognized as key regulators of vertebrate development, cell growth, and differentiation. Although nuclear receptors have held the attention since their discovery a decade ago, we report here on serine/threonine kinases as a new class of retinoid receptors. The conserved cysteine-rich domain of the NH2-terminal regulatory domains of cRaf-1, as well as several select domains of the mammalian protein kinase C (PKC) isoforms α, δ, ζ, and μ, the Drosophila and yeast PKCs, were found to bind retinol with nanomolar affinity. The biological significance was revealed in the alternate redox activation pathway of these kinases. Retinol served as a cofactor to augment the activation of both cRaf and PKCα by reactive oxygen, whereas the classical receptor-mediated pathway was unaffected by the presence or absence of retinol. We propose that bound retinol, owing to its electron transfer capacity, functions as a tag to enable the efficient and directed redox activation of the cRaf and PKC families of kinases.

Whereas retinoic acids control nuclear events, a second class of retinol metabolites, that is, the hydroxylated forms exemplified by 14-hydroxy-retro-retinol (HRR), operate primarily in the cytoplasm. They function as regulatory cofactors for cell survival/cell death decisions. In accordance with these biological aspects, we demonstrate that these retinoids bound protein kinase C (PKC) alpha with nanomolar affinity and markedly enhance the activation of PKC alpha and the entire downstream MAP kinase pathway by reactive oxygen species. HRR was 10 times more efficient than retinol, and the optimum doses are 10-7 and 10-6 M, respectively. PKC alpha activation was reversed rapidly by imposition of reducing conditions. The retinoid binding site was mapped to the first cysteine-rich region in the regulatory domain, C1A, yet was distinct from the binding sites of diacylglycerol and phorbol esters. The C1B domain bound retinoids poorly. The emerging theme is that retinoids serve as redox regulators of protein kinase C.

We examined nine chloramphenicol-resistant (minimal inhibitory concentration, greater than or equal to 15 micrograms/ml) Haemophilus influenzae strains isolated in various parts of the world to characterize the genetic and biochemical bases of the resistance; four were type b. All nine contained conjugative plasmids, ranging in molecular weight from 34 x 10(6) to 46 x 10(6), which encoded for resistance to chloramphenicol and tetracycline or chloramphenicol, tetracycline, and ampicillin. Deoxyribonucleic acid homology studies showed that these plasmids were closely related to a previously described ampicillin-resistant plasmid, RSF007, and to each other. All nine isolates and their chloramphenicol-resistant transconjugants produced chloramphenicol acetyltransferase. We conclude that chloramphenicol resistance in these strains of H. influenzae is via plasmid-mediated production of chloramphenicol acetyltransferase.

IgD is a surface immunoglobulin, which is coexpressed with IgM on >90% of mature B cells, but its levels in serum are extremely low compared to those of IgM. Its role as a surface receptor has been reemphasized by our recent findings that IgD receptors are induced on helper T cells by exposure to IgD and that such cells have immunoaugmenting properties. The present study shows that crosslinking of soluble IgD or of monomeric cell surface IgD is required and sufficient for the induction of T cells bearing receptors for IgD, both in vivo and in vitro. Effective IgD crosslinking in this respect can be obtained with antigen or with heterologous and immunogenic as well as nonimmunogenic allotype-specific anti-IgD. These results reinforce the concept that the induction of T cells bearing receptors for IgD is an integral component of the normal immune response. Recent studies in our laboratory have demonstrated the existence of helper T cells bearing receptors for IgD (T8) (1).This observation grew out of our earlier studies in which mice that bore the IgD-secreting TEPC-1017 or TEPC-1033 myeloma were found to generate significantly increased antibody responses to various antigens (2). Investigation of the cellular basis for this immunoaugmenting effect of IgD revealed that the elevated serum IgD levels in such mice stimulated the appearance of a significantly increased number of T cells that were capable of forming rosettes with IgD-coated sheep erythrocytes (SRBC) (3). Results from cell transfer studies confirmed that T cells exposed to IgD myeloma protein either in vivo or in vitro have a significant immunoaugmenting effect on antibody production in recipients (3, 4).Various methods of inducing TS cells were investigated. It was found that IgD receptors can also be induced in vivo by injection of antigen (1) or lymphokines, including interleukin 2 (IL-2) and interferon y (5). Exposure of helper T cells to such lymphokines in vitro also causes the expression of IgD receptors (5). In addition, resting T-cell clones of helper phenotype (L3T4+, Lyt-1', Lyt-2-), grown in the absence of IL-2, responded with increased IgD-receptor expression following overnight exposure to IgD or recombinant IL-2 (5). This effect was also seen when T-cell clones were stimulated with antigen in the absence of exogenously supplied IgD or IL-2, presumably as a result of autocrine secretion of lymphokines in response to antigen (5).Our findings that both T-dependent and T-independent antigens cause a small but significant increase in the frequency of TS cells within 5 days of antigen injection suggest that crosslinking of B-cell surface IgD by physiological stimuli induces TS cells. The present studies were carried out to explore further the role of IgD crosslinking in IgDreceptor induction. The findings show that T cells are induced to express receptors for IgD following exposure to IgD aggregated by anti-IgD. In addition, taking advantage of the availability of an IgD-secreting B-cell hybridoma, , we show that soluble monomeric IgD i...

Previous studies on murine T cell IgD-R have shown that these receptors recognize N-glycans of murine IgD, and not of other Ig isotypes. We have now studied the specificity of IgD-R on human T cells. Human IgD digested with proteinase K to fragments of X 5 kDa inhibit the ability of T cells to form rosettes with IgD-coated ox erythrocytes. The same amount of digested IgG does not. We tested all the human Ig isotypes: IgG1,-2,-3,-4, IgA2, IgE and IgM fail to inhibit significantly at 20 ? g/assay. However, IgA1 is as effective as IgD itself, showing approximately 60 % and 80 % inhibition at 5 ? g and 10 ? g/assay. Human IgA1 and IgD both contain Gal-1 1 3-GalNac-rich O-linked glycans, and on this basis are both bound to ricin and jacalin. The O-linked glycans may therefore also represent the common moiety binding to IgD-R. Disaccharides Gal-1 1 3-GalNac, and Gal-1 1 4-Glc at 10 ? g/assay blocked IgD rosetting while Gal-1 1 6-Glc did not. We conclude that the human IgD-R is a lectin, differing from the murine IgD-R in that it has both IgA1 and IgD as ligands.