Fate of human lactoferrin and myeloperoxidase in phagocytizing human neutrophils: effects of immunoglobulin G subclasses and immune complexes coated on latex beads

Human neutrophils (PMN) degranulated in response to soluble human immune complexes and to myeloma proteins, including subclasses of immunoglobulin G (IgG)₁, IgG₂, IgG₃, and IgG₄ coated on 1.09 μm latex beads. Immunochemical measurement of lactoferrin (LF) from specific granules and myeloperoxidase (MPO) from azurophil granules showed that both classes of granule degranulated. Beads with soluble complexes of human anti pigeon IgG normal pigeon IgG, prepared from serum of a patient with pigeon breeders disease, induced significantly greater degranulation than did pigeon IgG coated beads. Up to 40% of LF in the PMN degranulated during phagocytic challenge and 86% of that entered the extracellular fluid. Twenty to 30% of the MPO degranulated, but less than 50% of that entered the extracellular fluid. The degranulated LF and MPO which remained in the PMN were recovered from phagocytic vacuoles. Beads coated with purified human myeloma proteins (12 different ones, three of each subclass) induced degranulation in the order IgG₃>IgG₁>IgG₂>IgG₄; however, these differences were found to be a function of the amount of latex ingested. Thus, the amount of degranulation was dependent more on the opsonizing capacity of the immunoglobulins rather than on their intrinsic capacities for inducing degranulation. Degranulation of both LF and MPO in response to IgG subclasses followed patterns similar to those caused by soluble immune complexes, and IgG₃ coated on beads caused degranulation equal to that caused by human complex coated beads. Degranulation to IgG₃ and IgG₄ was uninfluenced by fresh compared with heat inactivated human AB serum. This was true although IgG₃ beads fixed greater than sixfold more complement than did IgG₄ beads. Evidently human IgG subclasses enhance phagocytosis and degranulation of human PMN. The overwhelmingly extracellular degranulation of LF in response to various bead coatings suggest that it subserves a major portion of its role outside PMN.