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SummaryThe IgG Fc receptor II on human monocytes is polymorphic in its ability to bind mIgG1, and its isoelectric focusing pattern. To study the molecular basis of this polymorphism, a cDNA library from cell line K562, expressing two different allelic forms (high responder [HR] and low responder [LR]) of FcyRII, was used for cDNA cloning. We report the isolation and identification of different FcyRII cDNA clones, comprising the LR form of FcyRII, as was evident from studies using a new HRspecific anti-FcyRII mAb 41H16, and from rosetting experiments . Sequence analysis revealed that HR and LR forms differ by two amino acids, both located in the external domain. In the cloned LR form, a glutamine is substituted by a tryptophan residue at as position 27, located in the first Ig-like domain, and an arginine residue by a histidine residue at as position 131 in the second Ig-like domain. Furthermore, an FcyRII cDNA clone was isolated with a deletion of 123 bp, overlapping the predicted transmembrane segment . Data showing the presence of an alternatively spliced mRNA detectedby using polymerase chain reaction (PCR) might suggest the existence of a soluble form of the human FcyRII, in addition to the membrane-bound forms .

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The interaction of CD40 ligand (CD40L) on activated T cells with CD40 on B cells, monocytes and dendritic cells is essential for humoral immunity and for up-regulation of antigen-presenting cell (APC) functions, as a result of signaling through CD40. There are also some indications that after interaction with CD40, CD40L can directly signal T cells. In this study we demonstrate that upon stimulation of human peripheral blood T cells through the T cell receptor (TCR)/CD3 complex, CD40/CD40L interaction strongly enhances the production of Th1 cytokines such as interleukin (IL)-2 and interferon (IFN)-gamma and Th2 cytokines such as IL-4, IL-5 and IL-10 by a direct effect on T cells. Furthermore, CD40/CD40L interaction synergizes with IL-12 in selectively enhancing IFN-gamma production by purified anti-CD3-stimulated T cells. These effects were observed at both the protein and the mRNA level. Both CD4+ and CD8+ T cells were able to produce IFN-gamma in the presence of helper signals from IL-12 and CD40, although CD8+ T cells were less active. Since CD40/CD40L interaction also up-regulates IL-12 production and B7 expression by APC, our results suggest that CD40/CD40L interaction is bidirectional, and promotes activation of both APC and T cells.

Abstract. Newly synthesized class II molecules of the major histocompatibility complex must be transported to endosomal compartments where antigens are processed for presentation to class II-restricted T cells. The invariant chain (Ii), which assembles with newly synthesized class II et-and [3-chains in the endoplasmic reticulum, carries one or more targeting signals for transport to endosomal compartments where Ii dissociates from a[3Ii complexes. Here we show that the transport route of aBIi complexes is regulated selectively by two forms of Ii (p33 and p35) that are generated by the use of alternative translation initiation sites. Using a novel quantitative surface arrival assay based on labeling with [6-3H]-D-galactose combined with biochemical modification at the cell surface with neuraminidase, we demonstrate that newly synthesized tx[3Ii molecules containing the Ii-p33 isoform can be detected on the cell surface shortly after passage through the Golgi apparatus/trans-Golgj network. A substantial amount of these a[3Ii complexes are targeted to early endosomes either directly from the trans-Golgi network or after internalization from the cell surface before their delivery to antigen processing compartments. The fraction of otl3Ii complexes containing the p35 isoform of Ii with a longer cytosolic domain was not detected at the cell surface as determined by iodination of intact cells and the lack of susceptibility to neuraminidase trimming on ice. However, treatment with neuraminidase at 37°C did reveal that some of the et13Ii-p35 complexes traversed early endosomes. These results demonstrate that a fraction of newly synthesized class II molecules arrive at the cell surface as etl3Ii complexes before delivery to antigen processing compartments and that class II al3Ii complexes associated with the two isoforms of Ii are sorted to these compartments by different transport routes.