Immunological responses to fed protein antigens in mice. I. Reversal of oral tolerance to ovalbumin by cyclophosphamide.

Feeding ovalbumin over a wide range of doses is known to reduce subsequent systemic immune responses to parenteral immunization. In the present study, we have fed mice 2 mg and 25 mg ovalbumin (OVA) 2 weeks before systemic immunization and followed the resulting humoral antibody and cell-mediated immune (CMI) responses. The results indicate that while 25 mg OVA will reduce subsequent IgM, IgG and CMI responses to OVA, feeding 2 mg OVA will only suppress CMI responses and to a lesser extent the IgM response. Furthermore, the tolerant state induced by feeding 25 mg OVA was only partially prevented by 100 mg/kg cyclophosphamide (CY) while the suppressed CMI after feeding 2 mg OVA was completely blocked by CY pretreatment. These findings suggest that the humoral and cell-mediated limbs of the immune response may be controlled by different regulatory systems after feeding antigen, and that activation of these systems is dependent on the dose of oral antigen use. In addition, the results are in agreement with our previous finding that CY pretreatment will allow the development of CMI in the gut and gut-associated lymphoid tissue (GALT) after oral OVA and suggest that this phenomenon is related to breakdown of oral tolerance induction.