CD4 and colitis: In this model, immunodeficient mice lacking mature T and B cells (e.g., Rag1 -/-) develop colitis a few weeks following the transfer of naïve CD4+ T cells (CD25negCD45RBhigh) unless a fraction containing Treg cells (e.g., CD45RBlow or CD4+CD25+FOXP3+) are co-injected (10, 178).