The gene for murine Ia-associated invariant (1i) chains (1131 and 1i41) was characterized by sequence analysis. The gene extends over -9 kb and is organized in nine exons. Exon 1 encodes the 5' untranslated region and the cytoplasmic segment, exon 2 the membrane spaniing segment and adjacent amino acids and exons 3-8 the extracytoplasmic portion of 1131. Putative promoter sequences were found upstream of the start of the coding sequence. Between exons 6 and 7 an additional, alternatively spliced exon 6b has been identified. This exon is spliced into the mRNA coding for the li-related Ii41 protein. Exon 6b encodes a cysteine-rich domain of 64 amino acids. It shows a remarkably high homology to the repetitive elements in thyroglobulin, a precursor for thyroid hormone. Based on this homology, it is suggested that this domain (TgR) in Tg and in 1141 may play a role either in hormone formation or as a carrier in the transport of molecules (thyroid hormone or processed antigen respectively) between intracellular compartments.

The invariant (Ii) chain is a membrane‐spanning glycoprotein found intracellularly associated with class II major histocompatibility complex (MHC) molecules. Using hybrid‐selected translation and the Ii‐specific monoclonal antibody In‐1, we have isolated a cDNA clone (pIi‐5) coding for most of the Ii chain. Sequence analysis of this clone reveals an open reading frame encoding 169 amino acid residues. The protein is rich in methionine and contains two potential N‐glycosylation sites. No stretch of uncharged amino acid residues, characteristic for a membrane‐spanning segment, is found close to the COOH‐terminal end. There is one, however, close to the NH2‐terminal end. As it is know that approximately 20 amino acid residues of Ii chain are exposed on the cytoplasmic side, we conclude that the Ii chain spans the membrane exposing the NH2 terminus on the cytoplasmic side and the COOH terminus on the luminal side.

The stimulatory as well as the inhibitory capacity of alloantisera has been investigated with respect to rat mast cell functions. Alloantibody against alloantigens coded for by the major histocompatibility (H-1) gene region promoted histamine release from purified LEW mast cells. This process was found to be complement-independent but demonstrated an absolute requirement for calcium. Pretreatment of mast cells with anti-H-1 antisera in the absence of calcium markedly suppressed the IgE-dependent histamine release challenged either by antigen or by anti-IgE antibody. The alloantisera, however, did not interfere with the ability of compound 48/80-associated histamine liberation. Additionally, antibodies specific for H-1 antigens were highly effective in inhibiting the binding of IgE to the mast cell surface. Alloantisera absorbed with erythrocytes lost their capacity to block mast cell functions. Based on these data the possible ralationship between H-1 alloantigens and the IgE receptor on the mast cell surface is discussed.

The chromosomal assignment of the gene encoding the invariant (Ii) chain associated with the mouse immune response antigens (Ia) was determined by Southern blot analysis of DNA from a panel of mouse X Chinese hamster somatic cell hybrids cleaved with Hind III or Eco RI. Using a mouse li cDNA as a hybridization probe, we localized the gene coding for the invariant chain to mouse chromosome 18.

Rat alloantibodies recognizing classical transplantation antigens (CTA) or non-H-1 determinants were able to compete effectively with monomeric IgE or IgG-coated sheep erythrocytes for receptor sites on the rat mast cell surface. Inhibitory capacity, however, was entirely confined to anti-CTA antibodies of the IgG2a subclass, whereas IgG1 antibodies lacked this ability. Analogously, F(ab')2 fragments of anti-CTA antibody consistently failed to affect IgE binding, but exposure of cell-bound F(ab')2 to anti-rat IgG restored its inhibitory capacity. From these results it was concluded that receptor sites recognizing the Fc portion of the anti-CTA molecule are involved in the inhibition process. Based on a cytotoxicity assay and on comparative absorption studies on alloantisera, the existence and relative amount of CTA and I region-associated antigens on purified rat mast cells and lymph node cells were analyzed. Whereas the CTA concentration per unit surface area on both cell types was very similar, rat mast cells consistently lacked Ia antigens.