CD19 and Splenomegaly: Notably, splenomegaly develops only in very old mice, likely due to strong counterregulation of CD30/CD30-L interactions in young and middle-aged CD30//CD19-Cre mice, where CD30 levels on the cell surface of B cells are low (as shown in Fig. 2D) and the expansion of CD30-L-expressing T cells is limited (Fig. 4B).