Five species of bacteria known to be naturally-occurring pathogens of cats were screened for their ability to grow in feline macrophages in vitro, and to induce
antibodies and delayed type
hypersensitivity (DTH) responses in vivo. Two of these organisms, L. monocytogenes and S. marcescens, were selected for further study based on clear-cut differences in their in vitro and in vivo behavior. Listeria was macrophage tropic, induced DTH, and evoked poor antibody responses post-recovery, whereas Serratia remained extracellular, did not induce a DTH reaction, and produced high titer of
antibodies. Young specific pathogen free cats were then inoculated subcutaneously into the drainage areas of the right and left popliteal and auricular lymph nodes with either L. monocytogenes or S. marcescens. Each of the four lymph nodes were then removed in sequence over a two week period, weighed, cultured for viable bacteria, and
RNA extracted for Th1/Th2
cytokine mRNA quantitation. Antibody responses and delayed type
hypersensitivity responses were also measured. Identical to pilot studies, cats infected with Serratia developed very high levels of antibody compared to Listeria infected cats but no DTH, while Listeria infected cats produced negligible or low titers of
antibodies and strong DTH. Immunity to Listeria occurred around 168 h post
infection as evidenced by the disappearance of living bacteria from the nodes, while immunity to Serratia took over 264 h. Pronounced lymph node
hyperplasia occurred in both
infections, but persisted longer for Serratia. Enlargement of Serratia infected nodes was associated with marked follicular, primary and secondary germinal center and medullary
hyperplasia. Germinal center formation in Listeria stimulated nodes was much less intense and dense accumulations of macrophages dissected between follicles downward from the subcapsular sinuses. Although functional and histologic studies showed a clear-cut cell-mediated vs. humoral response in the respective Listeria and
Serratia infections, preferential
cytokine mRNA upregulation was observed for only two of the five major Th1/Th2
cytokines measured.
Interferon-gamma, a Th1
cytokine, was much more elevated in the Listeria stimulated nodes, but
TNF-alpha (also a Th1
cytokine) was more elevated in Serratia infected nodes.
Interleukin-12, an important Th1
cytokine, was elevated to equal levels in both
infections as were the Th2
cytokines IL-4 and
IL-10.