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Clinical observations made on a patient with acute hypersensitivity pneumonitis revealed that the patient's serum contained a mitogenic inhibitor and an extremely high haptoglobin (Hp) level; this led to an investigation of the role of Hp in lymphocyte function. Hp was isolated and purified from acute phase rabbit serum by a new method using DE-52 anion exchange chromatography and preparative isoelectric focusing in a range of pH 3.0-5.0. This technique produced a homogenous, biologically active product in fewer steps and in higher yields than existing techniques. Purified rabbit Hp significantly inhibited polyclonal lymphocyte mitogenic responses to PHA or Con A in a dose-response fashion. Hp also significantly inhibited B-cell mitogenesis at a high concentration (200 mg/100 ml) in response to LPS while it enhanced B-cell mitogenesis at lower concentrations (50 and 100 mg/100 ml). Purified Hp had no effect on monoclonal, antigen-specific (BSA) mitogenesis. Rabbit alveolar macrophages produced significant amounts of prostaglandin E in vitro in response to LPS but Hp had no effect on this production. However, Hp alone caused approximately the same amount of stimulation of PGE production by alveolar macrophages as did LPS alone. The ability of Hp to modulate lymphocyte as well as macrophage function seems to indicate that Hp plays a role in the moderation of inflammation. This ability to moderate inflammation, especially in the lungs, may play an important role in regulating tissue damage and disease following inhalation of inflammatory aerosols.

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