In vivo T cell depletion in miniature swine using the swine CD3 immunotoxin, pCD3-CRM9

Background. Partially inbred miniature swine developed in this laboratory p rovide a unique preclinical large animal model for studying transplant tole rance. The importance of in vivo T cell depletion for establishing stable m ixed hematopoietic cell chimerism using a clinically relevant sublethal reg imen has been well documented in murine studies (1). Until now, the lack of an effective in vivo T cell-depleting reagent in swine has limited the pro gress of studies involving hematopoietic cell transplants. Methods. The swine CD3 immunotoxin, pCD3-CRM9, was prepared by conjugating our porcine-specific CD3 monoclonal antibody 898H2-6-15 to the diphtheria t oxin derivative, CRM9, The resultant immunotoxin was administered i.v, to s everal miniature swine at doses ranging from 0.15-0.2 mg/kg either in a sin gle dose or two doses 2 days apart. T-cell depletion was monitored in the p eripheral blood, mesenteric lymph node, and thymus by flow cytometric analy sis and histological examination. Results, T cells were depleted to less than 1% of their pretreatment levels based on absolute numbers in the peripheral blood. Fluorescence activated cell sorter analysis and histological examination of serial lymph node biop sies confirmed depletion of the CD3+ T cells rather than down modulation or masking of the surface CD3 expression. Depletion of the CD3 bright medulla ry thymocytes could also be detected by flow cytometry and histological exa mination after immunotoxin treatment. Conclusions. Administration of the immunotoxin i.v. drastically depletes ma ture T cells from the peripheral blood, lymph node, and thymus compartments of the pig. This first description of an effective in vivo T-cell depletin g reagent for the pig provides a valuable tool for studies of transplant to lerance in this large animal model. It also makes possible preclinical stud ies of T cell depletion with anti-CD3 immunotoxin in this large animal mode l.