B6.IDO−/− mice infected with LP-BM5 retrovirus succumbed to MAIDS as indicated by splenomegaly, serum hyper IgG2a and IgM, decreased responsiveness to B- and T-cell mitogens, conversion of a proportion of CD4+ T cells from Thy1.2+ to Thy1.2-, and increased percentages of CD11b+Gr-1+ cells. Here, IDO1 is linked to Splenomegaly.