Distribution of perforin‐containing cells in normal and pregnant mice*

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells elaborate a cytolytic protein named perforin or cytolysin. It was widely held that, in vivo, high quantities of perforin are not present in resting lymphocytes and are usually produced only by activated lymphocytes found under pathological conditions. Until now, only one tissue was known to synthesize abundant quantities of perforin under nonpathological conditions, the uterus during pregnancy. To investigate the possibility that perforin might also be synthesized by other tissues, several tissues besides the uterus from pregnant and normal mice were tested by immunofluorescence and immunoperoxidase for the presence of perforin. The tissues studied were the ears, brain, nasal epithelium, tongue, salivary gland, larynx, thymus, stomach, liver, spleen, small intestine, and lymph nodes; two cell populations with different sizes and levels of perforin expression were found. Large cells, displaying the NK cell phenotype and expressing high levels of perforin, were detected not only in the uterus but also in the salivary gland and lungs of pregnant mice. Small cells, expressing low levels of perforin, were detected mainly in the stomach and small intestine, and they were expressed in both pregnant and normal mice. Taken together, these results imply that perforin‐containing cells exist in vivo under nonpathological conditions, and that the immune system is endowed with heretofore unknown mechanisms for stimulating the activation of NK cells in a limited number of tissues during pregnancy.