Immunology B Cells Need Cleaved Li to Differentiate

The li chain is well characterized as an invariant subunit of the major histocompatibility complex (MHC) II in antigen-presenting cells. Matza et al. expand on earlier results to propose a mechanism for how li participates in B cell maturation. Matza et al. transfected human embryonic kidney 293 cells with green fluorescent protein (GFP)-tagged or epitope-tagged li proteins or protein fragments and showed that the localization of the tag varied depending on whether the tag was fused to the cytosolic NH₂-terminus or to the intralumenal COOH-terminus. NH₂-tagged proteins showed a cytosolic distribution for the tag, whereas COOH-tagged proteins appeared membrane-bound and associated with the endosomes. All of the fusion proteins had transmembrane domains and could be found in the membrane fraction. Western blot analysis with an antibody against the li cytosolic domain showed that the cytosolic domain was cleaved. This cleavage product was also detectable in primary B lymphocytes. Mutation of a previously identified cleavage site in the transmembrane domain of li prevented cleavage; mutation of a "d box" consensus degradation sequence stabilized the cleaved li fragment. By using an NF-B reporter construct, expression of the li cleavage product (either as just the fragment predicted to be produced by cleavage or as a transmembrane-containing cleavable fragment) stimulated reporter gene expression; whereas expression of the mutant that could not be cleaved did not stimulate reporter gene expression. Finally, in a B cell maturation model, expression of the cleavable or the cleavable and stabilized (mutated d box) forms of li stimulated B cell maturation; whereas the noncleavable mutant did not stimulate B cell maturation. Thus, B cell maturation appears to involve the intramembrane cleavage of li to produce a fragment capable of stimulating transcription through NF-B.

D. Matza, A. Kerem, H. Medvedovsky, F. Lantner, I. Shachar, Invariant chain-induced B cell differentiation requires intramembrane proteolytic release of the cytosolic domain. Immunity 17, 549-560 (2002). [Online Journal]

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