Two experimentally tested models.

(A) A putative autoantigen recognized by CD4⁺T cells is derived from GPI-anchored proteins. GPI⁺ cells (left) process GPI-anchored proteins and present antigenic peptides on MHC class II molecules, whereas GPI⁻ cells (right) do not present such antigenic peptides. (B) Some GPI-anchored protein is a ligand for some costimulatory molecules on CD4⁺ T cells. GPI⁺ cells (left) efficiently stimulate CD4⁺ T cells, whereas GPI⁻ cells (right) do not.