"Inactivation of the Ctla4 gene in mice, performed in the laboratories of Arlene Sharpe and Tak Mak, confirmed and further substantiated its negative regulatory role, since these mice developed very severe autoimmune disease associated with proliferating T cells (Tivol et al., 1995; Waterhouse et al., 1995)."
"In the first-in-man study MDX-010 was given to 9 patients in a phase I clinical trial at a single dose of 3 mg/kg and a response was observed in some melanoma patients (Hodi et al., 2003). The same year complete regression was reported in another trial in some treated melanoma patients, while severe autoimmune side effects were also observed (Phan et al., 2003)."
So I wouldn't take it "just in case" like a daily multivitamin. Plus, monoclonal antibodies are expensive!