Characterization of cytoplasmic lysozyme immunoreactivity as a histiocytic marker in normal canine tissues

The immunoreactive lysozyme distribution in normal canine tissues was determined to assess the value of lysozyme as a marker of histiocytic differentiation. Lysozyme was largely confined to monocytes, neutrophils, macrophages, serous cells in mucosal associated exocrine glands, and renal proximal tubular epithelium. Macrophages in the majority of tissues stained intensely for lysozyme, although in lymphoid tissues not all acid phosphatase and nonspecific esterase positive cells contained lysozyme. In particular, dendritic antigen presenting cells, including Langerhans cells, follicular dendritic cells, and interdigitating reticulum cells, lacked immunoreactive lysozyme; hence lysozyme appeared to represent a discriminatory marker with respect to these cells. Also, a small proportion of non-dendritic macrophages appeared to lack lysozyme. It was concluded that the demonstration of immunoreactive lysozyme was a useful adjunct to conventional morphological techniques for the identification of tissue macrophages provided that due caution was exercised in the interpretation of the results of lysozyme staining.