Activation of iNKT cells by a distinct constituent of the endogenous glucosylceramide fraction

Invariant natural killer T (iNKT) cells are a specialized T-cell subset that recognizes lipids as antigens, contributing to immune responses in diverse disease processes. Experimental data suggests that iNKT cells can recognize both microbial and endogenous lipid antigens. Several candidate endogenous lipid antigens have been proposed, although the contextual role of specific antigens during immune responses remains largely unknown. We have previously reported that mammalian glucosylceramides (GlcCers) activate iNKT cells. GlcCers are found in most mammalian tissues, and exist in variable molecular forms that differ mainly in N-acyl fatty acid chain use. In this report, we purified, characterized, and tested the GlcCer fractions from multiple animal species. Although activity was broadly identified in these GlcCer fractions from mammalian sources, we also found activity properties that could not be reconciled by differences in fatty acid chain use. Enzymatic digestion of beta-GlcCer and a chromatographic separation method demonstrated that the activity in the GlcCer fraction was limited to a rare component of this fraction, and was not contained within the bulk of beta-GlcCer molecular species. Our data suggest that a minor lipid species that copurifies with beta-GlcCer in mammals functions as a lipid self antigen for iNKT cells.