Fig. 2. MHCII-mediated presentation of cytosolically introduced protein is not the result of peptide regurgitation and subsequent bystander or cross presentation. IL2 secretion was monitored to determine specific TCR activation of the OVA-specific MHCII (H-2^b)-restricted T cell line 13.8, to C57BL/6 (H-2^b) and/or C3H/HeJ (H-2^k) macrophages cytosolically loaded with 10 mg/ml OVA (+Ag). Bystander presentation was assessed by incubating T cell line 13.8 with C3H/HeJ (H-2^k) macrophages cytosolically loaded with antigens (+Ag) along with unloaded (-Ag) C57BL/6 (H-2^b) macrophages. IL2-secretion was monitored by a proliferation assay as described in Materials and Methods.