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SummaryAntigen presentation and microbial killing are critical arms of host defense that depend upon cargo trafficking into lysosomes. Yet, the molecular regulators of traffic into lysosomes are only partly understood. Here, using a lysosome-dependent immunological screen of a trafficking shRNA library, we identified the Arf-like GTPase Arl8b as a critical regulator of cargo delivery to lysosomes. Homotypic fusion and vacuole protein sorting (HOPS) complex members were identified as effectors of Arl8b and were dependent on Arl8b for recruitment to lysosomes, suggesting that Arl8b-HOPS plays a general role in directing traffic to lysosomes. Moreover, the formation of CD1 antigen-presenting complexes in lysosomes, their delivery to the plasma membrane, and phagosome-lysosome fusion were all markedly impaired in Arl8b silenced cells resulting in corresponding defects in T cell activation and microbial killing. Together, these results define Arl8b as a key regulator of lysosomal cellular and immunological functions.

Murine invariant natural killer T (iNKT) cells provide cognate and non-cognate help for lipid and protein-specific B cells, respectively. However, the long term B cell outcome following cognate iNKT help is currently unknown. We show that cognate iNKT cell help resulted in a B cell differentiation program characterized by extrafollicular plasmablasts, germinal center formation, affinity maturation and a robust primary IgG antibody response that was uniquely dependent on iNKT-derived interleukin 21 (IL-21). However, cognate iNKT cell help did not generate an enhanced humoral memory response. Thus, iNKT cell cognate help for lipid-specific B cells induces a unique signature which is a hybrid of classic T-dependent (TD) and T-independent type 2 (TI-2) B cell responses.

By identifying the lipid LPC as an endogenous antigen, recognized by the invariant subset of human NKT cells, this study establishes a novel link between these immunoregulatory cells and an inflammatory lipid mediator.

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