A monoclonal antibody that inhibits the antimicrobial action of a 57 KD cationic protein of human polymorphonuclear leukocytes.

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Abstract
Two monoclonal antibodies (mAb) specific for epitopes of a 57,000 m.w., cationic antimicrobial protein (CAP57) purified from granules of human polymorphonuclear leukocytes (PMN) have been produced. Both were IgG1 mouse antibodies with typical heavy and light chain structure. The mAb reactive with CAP57 failed to react specifically with other heretofore defined PMN or serum proteins as shown by ELISA. Both mAb showed specific reactivity in Western blots with CAP57. One of these mAb (P1G8) inhibited the antimicrobial action of CAP57 by 50% at a ratio of 62.5 micrograms antibody per microgram CAP57. The other mAb, P2A5, had no inhibitory capacity for CAP57. Binding constants of the two mAb for the antigen were determined and were found to be virtually identical. Thus, the greater inhibitory capacity of P1G8 for bacterial killing by CAP57 was not directly related to binding strength of the mAb. Competition experiments showed that unlabeled P1G8 could compete as well against radiolabeled P2A5 as could unlabeled P2A5. In the reverse experiment, it was seen that P1G8 competed with radiolabeled P1G8 for CAP57 better than unlabeled P2A5. These findings could be due to two antibodies that recognize different but adjacent epitopes on CAP57, one of the epitopes (P1G8) being closer to structure(s) of the protein essential to its antimicrobial action. Immunocytochemical studies showed positive staining with both mAb. The reaction was restricted to the cytoplasm of peripheral blood PMN and was of a granular pattern. Other peripheral blood cells (which included red blood cells, eosinophils, monocytes, and lymphocytes) failed to bind either mAb.

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