In addition to the major population of infiltrating leukocytes recovered from inflamed rat central nervous system (CNS), all of which expressed high levels of leukocyte common antigen CD45, many cells were coisolated that were MRC OX42+ (complement receptor 3/CD1lb) but expressed low-to-moderate levels of CD45 and major histocompatibility complex (MHC) class I molecules. Most cells from normal CNS, in contrast, lay within this latter, CD45'1 population.From previous in sit immunohistochemical studies, the fortuitously isolated CD45"w cells were probably resident (ramified) microglia. Using irradiation chimeras, we show that resident microglia respond to inflammation by upregulating CD45, CD4, and MHC class I molecules with a minority of these cells increasing their expression of MHC class H molecules. A 3-to 4-fold increase in the number ofmicroglia isolated from inflamed CNS provided indirect evidence that the cells had proliferated. In normal CNS, a very small population of blood-derived CD451fth-expressing cells are present; most MHC class II expression is associated with these few cells and not with the resident microglia.

Major histocompatibility complex (MHC) class I molecules present peptides from degraded intracellular antigens to CD8+ T cells. These peptides are translocated in an ATP-dependent fashion into the lumen of the endoplasmic reticulum (ER) for binding to class I molecules by means of the MHC-encoded transporters associated with antigen processing, TAP1 and TAP2. These are members of a family of proteins containing an ATP-binding cassette and form heterodimers in the ER membrane. Defects in the genes encoding TAP1 or TAP2 account for impaired class I assembly and antigen presentation in several human and rodent cell lines. Whereas MHC class I molecules select peptides according to binding motifs, it is not clear to what extent the TAP1-TAP2 transporters have peptide sequence and length specificity. Previous studies of the rat MHC class I molecule RT1Aa, suggested a specific conveyance of peptides by rat TAP1-TAP2. Here we substitute the amino- and carboxy-terminal and the penultimate amino-acid residues of model peptides to show that these residues influence the efficiency of transport. Human TAP and rat TAPa translocated peptides with hydrophobic and basic C termini, whereas mouse TAP and rat TAPu preferred peptides with hydrophobic C termini. This pattern correlates with the predominant peptide binding profiles of mouse and human class I molecules.

Three rat hybridoma cell lines have been isolated which produce monoclonal antibodies identifying a noduleenhanced, soluble component of Pisum sativum root nodules. These antibodies each recognized a protease‐sensitive band (Mr 95K) on SDS‐polyacrylamide gels. The 95K antigen was resolved by isoelectric focusing into acidic and neutral components which were separately detected by AFRC MAC 236 and MAC 265 respectively. The third antibody (MAC 204) reacted with both acidic and neutral components through an epitope that was sensitive to periodate oxidation. These monoclonal antibodies were used for immunogold localizations at light and electron microscopic levels. In each case, the antigen was shown to be present in the matrix that surrounds the invading rhizobia in infection threads and infection droplets, as well as in the intercellular spaces between plant cell walls of nodules and also of uninfected roots. By contrast, a fourth monoclonal antibody, AFRC JIM 5, labelled a pectic component in the walls of infection threads, and JIM 5 was also found to label the middle lamella of plant cell walls, especially at three‐way junctions between cells. The composition and structure of the infection thread lumen is thus comparable to that of an intercellular space.

In mammalian cells, short peptides derived from intracellular proteins are displayed on the cell membrane associated with class I molecules of the major histocompatibility complex (MHC). The surface presentation of class I-peptide complexes presumably alerts the immune system to intracellular viral protein synthesis. Peptides derived from the cytosol must reach the cisternae of the endoplasmic reticulum where they are required for the assembly of stable class I molecules, and it has been proposed that the products of the two MHC-encoded ATP-binding cassette (ABC) transporter genes function to deliver the peptides across the membrane of the endoplasmic reticulum. This idea is supported by experiments in which transfection of a human cell line defective in class I expression with a complementary DNA of one of these genes restored cell surface expression levels. Here we show that the complete phenotype of the mouse mutant cell line RMA-S, in which lack of surface expression of stable class I molecules correlates with an inability to present viral peptides originating in the cytosol, is repaired by the cDNA of the other transporter gene. These results are consistent with the possibility that the two transporter polypeptides form a heterodimer.