Inhibitory Effects of Thymus-Independent Type 2 Antigens on MHC Class II-Restricted Antigen Presentation: Comparative Analysis of Carbohydrate Structures and the Antigen Presenting Cell

The role of thymus-independent type 2 (TI-2) antigens (polysaccharides) on the MHC-II-restricted processing of protein antigens was studiedin vitro.In general, antigen presentation is inhibited when both peritoneal and splenic macrophages (Mφ) as well as Küpffer cells (KC) are preincubated with acidic polysaccharides or branched dextrans. However, the inhibitory effect of neutral polysaccharides was minimal when KC were used as antigen presenting cells (APC). Morphological evaluation of the uptake of fluoresceinated polysaccharides clearly correlates with this selective and differential interference. Polysaccharides do not block MHC-I-restricted antigen presentation. Some chemical characteristics shared by different saccharides seem to be specially related to their potential inhibitory abilities: (i) those where two anomeric carbon atoms of two interlinked sugars and (ii) those containing several sulfate groups per disaccharide repeating unit. No polysaccharide being inhibitory in Mφ abrogated antigen processing in other APC: lipopolysaccharide-activated B cells, B lymphoma cells, or dendritic cells (DC). Using radiolabeled polysaccharides it was observed that DC and B cells incorporated less radioactivity as a function of time than Mφ. Morphological evaluation of these different APC incubated for extended periods of time with inhibitory concentrations of polysaccharides revealed intense cytoplasmic vacuolization in Mφ but not in B cells or DC. The large majority of Mφ lysosomes containing polysaccharides fail to fuse with incoming endocytic vesicles and delivery of fluid-phase tracers was reduced, suggesting that indigestible carbohydrates reduced the fusion of these loaded lysosomes with endosomes containing recently internalized tracers. It is suggested that the main causes of this antigen presentation blockade are (i) the chemical characteristics of certain carbohydrates and whether the specific enzymatic machinary for their intracellular degradation exists; and (ii) the different phagocytic abilities of distinct APC populations, fluid-phase pinocytosis and receptor-mediated saccharide uptake, and existence of a differential antigen-processing pathway in Mφ and DC or B cells, which could be based on a polysaccharide-inhibited step present in Mφ but unafected or irrelevant in both B cells and DC.