This study examines the in vitro response of human lamina propria lymphocytes (LPLs) to food antigens. LPLs were obtained from jejunum of healthy individuals undergoing gastric bypass operations for morbid obesity. Proliferation was assayed by [3H]thymidine incorporation and cytokine production by ELISA. LPLs proliferated in response to keyhole limpet hemocyanin, ovalbumin, lactoglobulin, and phytohemagglutinin (PHA), but not to yeast. The responses to lactoglobulin and PHA were inhibited by the CTLA4/Fc chimera and by MAbs against CD2, CD58, CD80, and CD86, indicating stimulation of CD28+ LPLs with antigen-presenting cells through activation of the CD2 pathway. Besides producing IL-2, IL-10, and TNF-alpha, LPLs synthesized large amounts of IFN-gamma (100 ng/ml) with lactoglobulin, a process dependent upon CD80/CD86, CD40/CD40L, and IL-12. After a three-day culture with lactoglobulin, ovalbumin, or PHA, LPLs developed suppressor activity that reduced proliferation of naive LPLs to these same stimuli. In summary, LPLs first respond to lactoglobulin by proliferation and IFN-gamma production, then by development of antigen nonspecific suppression.