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Abstract: Anti-Galα1–3Gal antibodies (antiαGal Ab) are a major barrier to clinical xenotransplantation as they are believed to initiate both hyperacute and acute humoral rejection. Extracorporeal immunoadsorption (EIA) with αGal oligosaccharide columns temporarily depletes antiαGal Ab, but their return is ultimately associated with graft destruction. We therefore assessed the ability of two immunotoxins (IT) and two monoclonal antibodies (mAb) to deplete B and/or plasma cells both in vitro and in vivo in baboons, and to observe the rate of return of antiαGal Ab following EIA. The effects of the mouse anti-human IT anti-CD22-ricin A (∝CD22-IT, directed against a B cell determinant) and anti-CD38-ricin A (∝CD38-IT, B and plasma cell determinant) and the mouse anti-human anti-CD38 mAb (∝CD38 mAb) and mouse/human chimeric anti-human anti-CD20 mAb (∝CD20 mAb, Rituximab, B cell determinant) on B and plasma cell depletion and antiαGal Ab production were assessed both in vitro and in vivo in baboons (n = 9) that had previously undergone splenectomy. For comparison, two baboons received nonmyeloablative whole body irradiation (WBI) (300 cGy), and one received myeloablative WBI (900 cGy). Depletion of B cells was monitored by flow cytometry of blood, bone marrow (BM) and lymph nodes (LN), staining with anti-CD20 and/or anti-CD22 mAbs, and by histology of LN. EIA was carried out after the therapy and antiαGal Ab levels were measured daily. In vitro ∝CD22-IT inhibited protein synthesis in the human Daudi B cell line more effectively than ∝CD38-IT. Upon differentiation of B cells into plasma cells, however, less inhibition of protein synthesis after ∝CD22-IT treatment was observed. Depleting CD20-positive cells in vitro from a baboon spleen cell population already depleted of granulocytes, monocytes, and T cells led to a relative enrichment of CD20-negative cells, that is plasma cells, and consequently resulted in a significant increase in antiαGal Ab production by the remaining cells, whereas depleting CD38-positive cells resulted in a significant decrease in antiαGal Ab production. In vivo, WBI (300 or 900 cGy) resulted in 100% B cell depletion in blood and BM, > 80% depletion in LN, with substantial recovery of B cells after 21 days and only transient reduction in antiαGal Ab after EIA. ∝CD22-IT depleted B cells by > 97% in blood and BM, and by 60% in LN, but a rebound of B cells was observed after 14 and 62 days in LN and blood, respectively. At 7 days, serum antiαGal IgG and IgM Ab levels were reduced by a maximum of 40–45% followed by a rebound to levels up to 12-fold that of baseline antiαGal Ab by day 83 in one baboon. The results obtained with ∝CD38-IT were inconclusive. This may have been, in part, due to inadequate conjugation of the toxin. Cell coating was 100% with ∝CD38 mAb, but no changes in antiαGal Ab production were observed. ∝CD20 mAb resulted in 100% depletion of B cells in blood and BM, and 80% in LN, with recovery of B cells starting at day 42. Adding 150cGy WBI at this time led to 100% depletion of B cells in the BM and LN. Although B cell depletion in blood and BM persisted for > 3 months, the reduction of serum antiαGal IgG or IgM Ab levels was not sustained beyond 2 days. ∝CD20 mAb + WBI totally and efficiently depleted CD20- and CD22-positive B cells in blood, BM, and LN for > 3 months in vivo, but there was no sustained clinically significant reduction in serum antiαGal Ab. The majority of antibody secretors are CD38-positive cells, but targeting these cells in vitro or in vivo with ∝CD38-IT was not very effective. These observations suggest that CD20-and CD22-positive B cells are not the major source of antiαGal Ab production. Future efforts will be directed towards suppression of plasma cell function. [ABSTRACT FROM AUTHOR]