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The aim of this study was to investigate whether I antigen occurs in association with Mycoplasma pneumoniae in a form that may be immunogenic during natural infection or experimental immunization. I antigen activity was detected by radioimmunoassay in suspensions of M. pneumoniae MY11965 and in the soluble phase of mycoplasma lysates prepared with Triton X-100. There was evidence for the occurrence of I antigen in at least two macromolecular forms. The first form partitioned in the lipid phase following chloroform-methanol extraction and chromatographed on thin-layer chromatograms as a ceramide decasaccharide. The second form was associated with the residue after lipid extraction and was solubilized by treatment with sodium dodecyl sulfate or pepsin; this component was tentatively designated a glycoprotein or polysaccharide and was not investigated further. In a lipid extract from mycoplasmas that had been surface labeled by the galactose oxidase-NaB3H4 method, two 3H-labeled glycolipids were detected as minor components which chromatographed on thin-layer chromatograms in the region of an authentic I-active ceramide decasaccharide. However, no significant radioactivity was incorporated into glycolipids after metabolic labeling with [3H]glucosamine. These observations suggested that the mycoplasmas contained surface-associated glycolipids with I antigen activity that were of exogenous origin. This was supported by the observations that horse, rabbit, and fetal calf sera contained I antigen activity and that the I antigen activity in M. pneumoniae cultures reflected the levels found in the sera included in the culture media. From rabbit serum, which expressed the highest antigen activity, an I-active glycolipid was isolated that chromatographed as a ceramide decasaccharide. I-active substances passively adsorbed onto M. pneumoniae are potentially immunogenic. However, we consider these unlikely to be the main stimulus for autoantibody production in natural infection, since the autoantibodies elicited are restricted to the I carbohydrate antigen and there is a lack of antibodies to other glycolipids that may be adsorbed from serous and cellular components of the host tissues. In our view, the more likely stimulus is the specific complex formed between the mycoplasma and the sialo-oligosaccharide receptors of the Ii antigen type, as suggested previously.

Evidence for occurrence of passively adsorbed I antigen activity on a cultured strain of Mycoplasma pneumoniae