Eur. J. Immunol. 1994. zyxwvutsrq 24: 2755-2760 Surface IgM-mediated specific Ag presentation 2755 z Ko-Jiunn LiuO, Vandana S. Parikh., Philip W. Tucker. and Byung S. Kimn Department of Microbiology-Immunology, and Tumor Cell Biology Program" Northwestern University Medical School, Chicago and Department of Microbiology, University of Texas Southwestern Medical Center., Dallas Surface immunoglobulins mediate efficient transport of antigen to lysosomal compartments resulting in enhanced specific antigen presentation by B cells* A BCLl immunoglobulin (Ig) transfectant, expressing wild-type surface (s)IgM with the TEPC-15 idiotype (T15-Id) and anti-phosphorylcholine (PC) specificity, was previously shown to present PC-conjugated hen egg-white lysozyme (PC-HEL) to a HEL-specific Tcell hybridoma at a lower antigen (Ag) concentration than that required for native €EL. Two variant Ig transfectants, expressing T15-Id sIgM with substitutions either in the entire spacer, transmem- brane (TM) domain and cytoplasmic tail (B186 variant) or in the NH2-terminal third of TM domain only (TM2 variant), failed to display this sIgM-mediated, enhanced presentation of PC-HEL at low concentrations. However, prolonged treatment with anti-T15-Id monoclonal antibody (mAb) led to a reduction of surface expression of theTl5-Id sIgM in the wild-type and TM2 variant, but not in the B186 variant sIgM transfectants. Treatment with anti-T15-Id mAb also resulted in an increased intracellular accumulation of T15-Id sIgM in the wild-type transfectant, but not in the B186 variant. Subcellular fractionation analysis revealed that the ligands bound to the T15-Id sIgM are not efficiently transported to the dense lysosomal compartments in both B186 and TM2 transfectants, as compared to the wild-type sIgM transfectant. A significant increase in tyrosine phosphorylation after cross-linking of the T15-Id sIgM was observed only in the wild-type sIgM transfectant.These results suggest that, while the NHzterminal third of theTM region is not involved in the process responsible for the ligand-induced reduction of surface expression of sIgM, it appears to be essential for subsequent transport of sIgMAigand complexes to the lysosomal compartments, as well as efficient activation of tyrosine kinases. These results strongly suggest that sIg-mediated enhancement of specific antigen presentation reflects the ability of sIg to efficiently transport antigen to the lysosomal compartments, and possibly the activation of protein tyrosine kinases. 1 Introduction The B cell-Ag receptor complex consists of the surface (s)Ig and at least two associatedmolecules, Ig-a and Ig-fi [l].The sIg serves to recognize and bind the specific Ag, whereas Ig-dIg-fi appear to be responsible for transducing signals following cross-linking of the sIg [l-41. After binding of specific Ag to the slg, this AgIsIg complex is internalized and transported to the endosomalllysosomal compartments where the Ag is processed for subsequent presentation to T cells. It has been shown that specific Ag is presented to T cells at a lower Ag concentration than that required for [I 130571 zyxwvu * This work was supported by USPHS research grants, RO1 A115446 (B.S.K.) and R01 A118016 (W.T.). Correspondence: B yung S. Kim, Department of Microbiology- Immunology, Northwestern University Medical School, 303 E. Chicago Ave., Chicago, IL 60611, USA Abbreviations: PC: Phosphorylcholine TSId TEPC-15 idiotype HEL: Hen egg-white lysozyme TM: Transmem- brane Key words: Surface immunoglobulins zyxwvuts I Antigen presentation I Intracellular transport I Antigen-antibody complexes I B cells nonspecific Ag by B cells carrying sIg for the specific Ag [5, 61. Although cross-linking of sIg by specific Ag also causes a cascade of signaling events, it is still not clear whether this sIg-mediated enhancement of Ag presenta- tion is simply a direct result of increased Ag uptake or involves more complicated intracellular signaling pro- cesses. Previously,we [7] and others [8] have shown that the transmembrane (TM) domain and cytoplasmic tail of sIg are important in such efficient presentation of specific Ag by B cells. It has been proposed that theTM region may be involved in the potential interactions of sIg with Ig-dIg-fi [l, 4, 91, or with other unidentified sIg-associated mole- cules. A recent report [lo] further indicates that the association of Ig-dIg-fi with sIg may be essential for the sIg-mediated enhancement of Ag presentation by Ag- specific B cells. We have previously shown that a BCLl line transfected with wild-type T15-Id sIgM with anti-PC specificity, is able to present PC-HEL more efficiently to T cells than native HEL [7]. Several variant T15-Id sIgM transfectants were also generated to investigate the role of the TM region in the enhanced Ag presentation mediated by sIg. These variant transfectants express T15-Id sIgM substituted, to various degrees, in the spacer, TM and cytoplasmic regions with equivalent regions of the I-Aa chain. All variant sIgM transfectants failed to demonstrate preferential presenta- tion of specific Ag (PC-HJZL). A reduction in the surface 0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1994 OO14-2980/94/1111-2755$10.OO + .25/0