CD4 and Autoimmunity: Overall, the RIP-HA model offers unique opportunities to study mechanisms of antigen-specific suppression of β cell autoimmunity, employing cotransfer of TCR-HA+ Treg cells, either with a Foxp3+ or Foxp3− phenotype, together with pathogenic T effector cells (CD4+TCR-HA+ or CD8+CL4+).