WASHINGTON (AP) — The Obama administration argued Monday that Guatemalans unknowingly exposed to sexually transmitted diseases by U.S. researchers in the 1940s cannot sue the United States, no matter how shameful and unethical the studies were.In its first response to a lawsuit filed on behalf of the experiment's subjects, the Justice Department late Monday said sovereign immunity protects federal health officials from litigation stemming from the study. The experiment conducted in the 1940s exposed Guatemalan prostitutes, prisoners, mental patients and soldiers with STDs to test the effects of penicillin. The studies were conducted without the test subjects' consent.