Defects within the innate immune system sensitize NF-kappaB-deficient (p50(-/-); p65(+/-)) mice to Helicobacter hepaticus (Hh)-induced colitis. Because IL-10 plays a central role in the inhibition of Hh-induced colitis, we hypothesized that the ability of IL-10 to inhibit the innate inflammatory response to Hh may be compromised in NF-kappaB-deficient mice. To test this hypothesis, we evaluated the ability of an IL-10-Ig fusion protein with IL-10-like properties to inhibit Hh-induced colitis in RAG-2(-/-) (RAG) and p50(-/-); p65(+/-); RAG-2(-/-) (3X/RAG) mice. As expected, IL-10-Ig efficiently inhibited the development of colitis in RAG mice. In contrast, the ability of IL-10-Ig to inhibit colitis was compromised in 3X/RAG mice. The defect in response to IL-10-Ig appeared to be primarily the result of the absence of the p50/p105 subunit, because the ability of IL-10-Ig to inhibit colitis was also compromised in p50(-/-); RAG-2(-/-) (p50/RAG) mice. Radiation chimeras demonstrated that the presence of p50/p105 within hemopoietic cells of the innate immune system was necessary for efficient inhibition of colitis by IL-10-Ig. Consistent with a defect in the suppressive effects of IL-10 in the absence of p50/p105, we found that the ability of IL-10 to control LPS-induced expression of IL-12 p40 was significantly compromised in macrophages lacking p50/p105. These results suggest that the absence of the p50/p105 subunit of NF-kappaB within hemopoietic cells of the innate immune system interferes with the ability of IL-10 to suppress inflammatory gene expression and Hh-induced colitis.