Alteration of intestinal microbial ecology in mice by recombinant interleuken-2

In addition to its beneficial immunostimulatory properties, interleukin-2 (IL-2) has many significant side effects including gastrointestinal (GI) toxicities. Preliminary studies of the effects of IL-2 on GI bacterial translocation suggested that IL-2 altered intestinal bacterial population levels. In order to further define the effects of IL-2 on intestinal microecology, groups of specific pathogen-free C57BL/6 mice were injected subcutaneously twice daily for 5 days with various dosages of human recombinant IL-2 (up to a maximum of 1.6 mg/kg injection) or an equal volume of sterile buffer. One day after the final injections, IL-2 had significantly (P<.05) increased in a dose-dependent fashion the mean cecal population levels of indigenousEscherichia coli, other gram-negative bacilli, streptococci, and staphylococci. Maximum increases above control cecal population levels were more than 10,000-fold forE. coli and streptococci, 209-fold for gram-negative bacilli other thanE. coli, and 93-fold for staphylococci. These changes were completely reversible, with normal cecal population levels 12 days after the last IL-2 injection. IL-2 did not change the total cecal population levels of strictly anaerobic bacteria. Ileal and cecal structures, as assessed by light microscopy, were not altered by the highest dose of IL-2. When incubated in vivo with 1.6 mg/ml of IL-2, the growth of an indigenous strain ofE. coli was not different from growth in broth alone. Thus, IL-2 treatment caused the intestinal overgrowth of common opportunistic aerobic and facultatively anaerobic bacterial pathogens, without altering total population levels of strict anaerobes or affecting intestinal histology.