Chapter 23. Monitoring Antigen Presentation and T-Cell Activation
Introduction
Analyze the Data 23-1: Monitoring Antigen Presentation and T-Cell Activation
To understand how ovalbumin (a protein from chicken eggs) and other foreign antigens in the cytoplasm of a cell are presented for immunosurveillance, ovalbumin can be introduced by electroporation into the cytoplasm of mouse B lymphocytes in primary cell culture. The ovalbumin is cleaved, and one of its cleavage products is a peptide with the sequence SIINFEKL (single-letter abbreviations). When the B cells are mixed with a clonal T-cell population that specifically recognizes SIINFEKL in the context of MHC molecules, the T cells become stimulated. The graphs below show the secretion of IL-2 after these T cells were mixed with B cells that had been treated in various ways.
Graph A: B cells were electroporated either with ovalbumin (solid line) or with a control protein (dashed line) not recognized by the clonal T-cell population.
Graph B: B cells were first incubated with an inhibitor of lysosomal cysteine proteases (solid line) or an inhibitor of the proteasome (dashed line), then electroporated with ovalbumin.
Graph C: B cells were exposed to a high concentration of the SIINFEKL peptide and then immediately fixed [killed] (dotted line) with formaldehyde or incubated for 2 hours in the presence (dashed curve) or absence (solid curve) of the proteasome inhibitor and then fixed with formaldehyde.
a. Under what conditions is IL-2 secreted? Is it likely that the T cells or the B cells are secreting IL-2?
Because, as shown in graph C, IL-2 is secreted in panel C and the B cells are killed, secretion of IL-2 must be from the T cells and would occur when the T cells are activated by the B cells. Activation would occur when the T cells recognize the SIINFEKL peptide, as the cells electroporated with the control protein (graph A) do not induce T-cell activation. Interestingly, the T cells are activated when SIINFEKL is presented to them on the surface of fixed cells (graph C, dashed and solid curves). Thus, B cells need not be alive, they just need to properly present antigen.
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