The subcellular localization of defensins and myeloperoxidase in human neutrophils: immunocytochemical evidence for azurophil granule heterogeneity

We used immunogold electron microscopy to examine the distribution of human defensins 1-3 (HNP1-3) and myeloperoxidase (MPO) in the cytoplasmic granules of thin-sectioned, normal human neutrophils. The presence of MPO, defensins, or both was noted in 388 (62.3%) of 623 granule profiles. Whereas MPO and defensins colocalized in 32.5% of such granules, 32.7% demonstrated only MPO and 34.8% demonstrated only defensins. The staining densities of MPO and HNP1-3 showed a low correlation, even when both molecules were present in the same granules (r = 0.09), indicating that they were not present in fixed relative proportions even when colocalized. The low partial correlation coefficient (r = -0.23) between MPO and HNP1-3 for the 388 labeled granules controlling for granule area suggested that the deposition of MPO and defensins might even be negatively correlated. The distribution of MPO and defensins within the cytoplasmic granules of human neutrophils followed a "non-colocalization and multi-compartmental" model that conflicts with the classical notion of azurophil granules as a homogeneous population of organelles marked by the presence of MPO.