Haptoglobin phenotyping by newly developed monoclonal antibodies. Demonstration of haptoglobin uptake into peripheral blood neutrophils and monocytes

We have generated two IgG murine mAbs that recognize native human haptoglobin (Hp). These mAbs, 3A8 and 4B2, efficiently bind to the Hp complex regardless of the serum donor's phenotype. The specificity of mAb 3A8 was confirmed by immunoaffinity purification of 3A8-binding material from human serum and subsequent N-terminal amino acid sequencing of the invariant 40-kDa chain. mAb 3A8 and 4B2 were also reactive with cell-associated Hp when studied by immunocytochemistry. When human peripheral blood leukocytes were tested, 90% of the granulocytes and a lesser (and variable) fraction of monocytes displayed an intense intracytoplasmatic granular staining. This was confirmed by flow cytometric analysis of permeabilized leukocytes and by demonstrating the presence of Hp (of the expected Hp serum phenotype) in extracts of washed granulocytes by immunoblotting. Leukocytes obtained from a Hp phenotype 2-2 donor, incubated in culture medium supplemented with 10% serum from a donor possessing the Hp 1-1 phenotype, contained both Hp phenotypes when analyzed by immunoblotting after a 6-h incubation period. In addition, Hp was actively exocytosed by granulocytes following their exposure to Candida albicans. These observations suggest that exogenous Hp is concentrated within granulocytes and not synthesized de novo and is, in turn, exocytosed following neutrophil activation. Northern blotting analysis is consistent with the lack of haptoglobin gene transcription in granulocytes. These findings together with the earlier observations that Hp modulates granulocyte activity suggest that Hp levels may be enhanced locally at sites of inflammation to modulate granulocyte activity.