© 2019 Elsevier B.V. Background: Measurement of serum anti-pig antibodies is an important parameter in immune monitoring after pig-to-nonhuman primate xenotransplantation. Pig aortic endothelial cells (pAECs) are commonly used for this purpose. However, human (h) CTLA4-Ig (abatacept/belatacept) could bind to pCD80/86 on the cells, and a secondary antibody (i.e., anti-human IgG) may recognize hCTLA4-Ig (in the absence of serum anti-pig IgG antibody binding to pAECs), potentially leading to misinterpretation of the results. Our aim was to determine whether hCTLA4-Ig binding to pAECs is associated with false-positive results. Methods: Sera were obtained from (i) naïve baboons (n = 3) and (ii) baboons (n = 2) that had undergone pig artery patch transplantation with/without hCTLA4Ig therapy. Serum IgM and IgG binding to (i) AECs, (ii) red blood cells (RBCs), and (iii) CD3+T cells in peripheral blood mononuclear cells (PBMCs) from an α1,3-galactosyltransferase gene-knockout pig expressing human CD46 (GTKO/hCD46) was measured by flow cytometry in the presence or absence of hCTLA-4Ig. Complement-dependent cytotoxicity (CDC) of wild-type (WT) pAECs by hCTLA4Ig was measured by flow cytometry. Results: Sera containing hCTLA4-Ig demonstrated significantly increased IgG (but not IgM) binding to pAECs (relative geometric mean [rGM] = 1.8) compared to sera without hCTLA-4Ig (rGM =1.3) (p < .01). In contrast, there was no increased binding to pRBCs or CD3+T cells. hCTLA4-Ig did not result in cytotoxicity of WT pAECs. Conclusions: pAECs might not be an optimal cell to investigate anti-pig IgG binding when hCTLA4-Ig is administered to the recipient, as a false-positive result may result from hCTLA4-Ig binding to the pAECs. CD3+T cells would be preferable targets (compared to pRBCs) because they express both carbohydrate and MHC class I/II antigens.