Clearance of mobilized porcine peripheral blood progenitor cells is delayed by depletion of the pragocytic reticuloendothelial system in baboons

Introduction. Attempts to achieve immunological tolerance to porcine tissue s in nonhuman primates through establishment of mixed hematopoietic chimeri sm are hindered by the rapid clearance of mobilized porcine leukocytes, con taining progenitor cells (pPBPCs), from the circulation. Eighteen hours aft er infusing 1-2x10(10) pPBPC/kg into baboons that had been depleted of circ ulating anti-alpha Gal and complement, these cells are almost undetectable by flow cytometry. The aim of the present study was to identify mechanisms that contribute to rapid clearance of pPBPCs in the baboon. This was achiev ed by depleting, or blocking the Fc-receptors of, cells of the phagocytic r eticuloendothelial system (RES) using medronate liposomes (MLs) or intraven ous immunoglobulin (Mg), respectively. Methods. Baboons (preliminary studies, n=4) were used in a dose-finding and toxicity study to assess the effect of MLs on macrophage depletion in vivo . In another study, baboons (n=9) received a nonmyeloablative conditioning regimen (NMCR) aimed at inducing immunological tolerance, including splenec tomy, whole body irradiation (300 cGy) or cyclophosphamide (80 mg/kg), thym ic irradiation (700 cGy), T-cell depletion, complement depletion with cobra venom factor, mycophenolate mofetil, anti-CD154 monoclonal antibody, and m ultiple extracorporeal immunoadsorptions of anti-aGal antibodies. The baboo ns were divided into three groups: Group 1 (n=5) NMCR+pPBPC transplantation ; Group 2 (n=2) NMCR+ML+pPBPC tranplantation; and Group 3 (n=2) NMCR+IVIg+p PBPC transplantation. Detection of pig cells in the blood was assessed by f luorescence-activated cell sorter and polymerase chain reaction (PCR). Results. Preliminary studies: ML effectively depleted macrophages from the circulation in a dose-dependent manner. Group 1: On average, 14% pig cells were detected 2 hr postinfusion of 1x10(10) pPBPC/kg. After 18 hr, there we re generally less than 1.5% pig cells detectable. Group 2: Substantially hi gher levels of pig cell chimerism (55-78%) were detected 2 hr postinfusion, even when a smaller number (0.5-1 x 10(10)/ kg) of pPBPCs had been infused , and these levels were better sustained IS hr later (10-52%). Group 3: In one baboon, 4.4% pig cells were detected 2 hr after infusion of 1x10(10) pP BPC/kg. After 18 hr, however, 7.4% pig cells were detected. A second baboon died 2 hr after infusion of 4 x 10(10) pPBPC/kg, with a total white blood cell count of 90,000, of which 70% were pig cells. No differences in microc himerism could be detected between the groups as determined by PCR. Conclusions. This is the first study to report an efficient decrease of pha gocytic function by depletion of macrophages with MLs in a large-animal mod el. Depletion of macrophages with MLs led to initial higher chimerism and p rolonged the survival of circulating pig cells in baboons. Blockade of macr ophage function with IVIg had a more modest effect. Cells of the RES, there fore, play a major role in clearing pPBPCs from the circulation in baboons. Depletion or blockade of the RES may contribute to achieving mixed hematop oietic chimerism and induction of tolerance to a discordant xenograft.