The significance of immunofluorescent immunoglobulin inclusions in polymorphonuclear leucocytes for the detection of circulating immune complexes.
Cytoplasmic inclusions of immunoglobulins and complement, detected by fluorescent antibodies in polymorphonuclear leucocytes (PMNs) that have been incubated with sera of certain patients, are considered to represent immune complexes (IC). The usefulness of this test--the indirect PMN phagocytosis test (IPPT)--for the detection of circulating IC was investigated using preparations of free and heat-aggregated immunoglobulins. Free IgG and IgM were phagocytozed by PMNs at high concentrations only, while free IgA was not phagocytozed at all. Normal human serum slightly enhanced the uptake of free IgG and IgM, but not of free IgA. Aggregates of IgG and IgA underwent phagocytosis at low concentrations, but IgM aggregates were not taken up more readily than free IgM. The uptake of IgG aggregates decreased in the presence of serum, while there was no influence upon the phagocytosis of IgA aggregates. Phagocytosis of C3 occurred only with IgG aggregates. In the presence of aggregates of IgG or IgA the phagocytosis of free immunoglobulins of other classes, in particular IgM, increased. The results of the IPPT for patients' sera showed that inclusions of C3 were found more frequently in combination with IgA or IgM than with IgG. Comparison with the 125I-Clq binding assay and the anti-IgA inhibition binding assay disclosed significant correlation between the phagocytosis of IgG and the precipitation of 125I-Clq and between the phagocytosis of IgA and the results of the anti-IgA inhibition binding assay. The PMN phagocytosis test may be useful for the detection of IgG and IgA containing IC but inclusion of IgM and C3 should be interpreted with some reserve.