HLA‐D region α‐chain monoclonal antibodies: Cross‐reaction between an anti‐DP α‐chain antibody and smooth muscle

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Abstract
Two monoclonal antibodies, Tal 3C3 and Tal 1 B5, raised against purified plasma membrane extract of the Bristol 8 B lymphoblastoid cell line and previously shown to recognize epitopes on denatured HLA‐D α‐chains revealed several differences in staining patterns when screened by immunocytochemical techniques on pre‐fixed paraffin‐embedded sections of human tissues. Although Tal 3C3 staining was weaker and more restricted than Tal 1 B5 on immunoregulatory cells expected to express class II antigens, this monoclonal antibody showed additional strong activity on normal smooth muscle. Biochemical studies show that whereas Tal 1B5 reacts with DR a‐chains, Tal 3C3 predominantly defines DP a‐chain subunits and a high molecular weight (200–250 kD) actin binding protein known as filamin. In view of several recent reports that the cellular distribution of class II antigens may be more widespread than hitherto thought and may be expressed by cells not normally involved in specific immune functions, these findings emphasize that unexpected monoclonal antibody reactions must be interpreted with caution. It is clearly important to include both immunohistological tissue section and biochemical studies in the initial screening procedures for monoclonal antibodies in order to distinguish between specific target antigen activity and cross‐reactions with unrelated molecules.