The invariant chain (Ii), a nonpolymorphic glycoprotein that associates with the immunoregulatory Ia proteins encoded by the major histocompatibility complex, has a proteoglycan form (Ii-CS) that bears a chondroitin sulfate glycosaminoglycan. In this proteoglycan form, Ii may remain associated with Ia at the cell surface. Inhibitors that prevent

The cellular and subcellular localization of cathepsin D, an aspartyl endopeptidase, was investigated in the central and peripheral nervous systems of the rat by light and electron microscopic immunocytochemistry. The reaction of rabbit anti-rat brain cathepsin D within ventral cervical spinal cord, cerebellum, corpus callosum, caudate nucleus, optic nerve, trigeminal ganglion, fifth cranial nerve and sciatic nerve was localized with an indirect immunoperoxidase technique. A number of tissue processing methods were utilized, but only in tissues fixed in paraformaldehyde-lysine-periodate and sectioned at thicknesses of 25-50 micron could antibody penetration, enzyme protein immunoreactivity and intact morphology be reliably attained. Immunoreactive cathepsin D was present in lysosomes and pleomorphic dense bodies of neurons in the anterior horn of spinal cord, cerebellar Purkinje and granule cell layers, caudate nucleus and trigeminal ganglion. Lysosomal localization of cathepsin D was also documented in oligodendrocytes, astrocytes, endothelial cells and Schwann cells. Reaction product was not observed in microglia although its presence there would be expected. With these methods, reaction product was not detected in the Golgi saccules of any cell type.

C3H/He and C57B1/6 mice were inoculated with 500 Trypanosoma cruzi trypomastigotes (Strain Y). During the acute phase infected mice presented parasitemia and enlargement of lymph nodes and spleens and intracellular parasites were observed in the heart. Examinations of cells derived from spleen and lymph nodes showed increased numbers of IgM and IgG-bearing cells. During the peak of splenomegaly, about day 17 post-infections, splenic lymphocytes showed a marked decrease in responsiveness to T and B-cell mitogens, parasite antigens and plaque forming cells (PFC) to sheep red blood cells (SRBC). Unfractionated or plastic adherent splenic cells from mice, obtained during the acute phase were able to suppress the response to mitogens by lymphocytes from uninfected mice. During the chronic phase. Disappearance of parasitemia and intracellular parasites in the hearts as well as a decrease in spleen size, was observed. These changes preceded the complete recovery of responsiveness to mitogens and T. cruzi antigens by C57B1/6 splenic lymphocytes. However, this recovery was only partial in the C3H/He mice, known to be more sensitive to T. cruzi infection. Partial recovery of humoral immune response also occurred in both strains of mice during the chronic phase.

The invariant chain (Ii) is a nonpolymorphic glycoprotein that associates with the Ia alpha- and beta-chains of MHC class II Ag during their transport to the cell surface. Although surface expression of Ia can occur in the absence of Ii, it has not been shown whether the intracellular association of Ia and Ii affects the biosynthetic rate or specificity of posttranslational modifications to the individual molecules. Analysis of transfected cell lines carrying either Ia, Ii, or both Ia and Ii demonstrated efficient assembly of alpha-beta whether or not Ii was present. Pulse-chase studies and two-dimensional gel electrophoresis showed that Ii did not affect the addition of Ia N-linked oligosaccharide chains, or the extent or rate of their conversion to the complex form, nor did it affect the sulfation of alpha and beta glycoproteins. Ii also did not affect the rate of Ia synthesis or appearance of Ia at the cell surface. In contrast, Ia dramatically affected the posttranslational modification of Ii. Although invariant chain was modified by addition of fatty acid, N-linked oligosaccharide, and glycosaminoglycan in the absence of Ia, the processing of Ii-linked oligosaccharide into more acidic, terminally glycosylated forms was significantly less when Ia was absent, and although conversion to proteoglycan did occur, the glycosaminoglycan chains were significantly shorter than normal. The disappearance of radiolabel from the 31,000 Da form of Ii was faster when Ia was present, and the processing of Ii was more rapid when it was associated with Ia. Thus, the rate and manner in which Ii enters and passes through the Golgi is critically affected by Ia.

Labeling of murine spleen cells with [3H]palmitate followed by analysis of immunoprecipitated Ia molecules indicated that Ia alpha- and beta-chains and their associated invariant chain contain covalently bound fatty acid. This modification is present in I-A and I-E molecules and has been found in all haplotypes examined. The 3H label was not dissociated from the glycoproteins by detergents or under the denaturing conditions of SDS-polyacrylamide gel electrophoresis. The fatty acid linked to Ii is released by treatment with neutral hydroxylamine, which indicates thioester linkage. The acylation of alpha- and beta-chains appears to involve attachment of palmitoyl groups via an ester linkage sensitive to alkaline hydrolysis. The radioactive species released from the isolated chains by treating with KOH/methanol co-migrated with palmitic acid and palmitic acid methyl ester on thin-layer chromatography.