Macrophages play a crucial role in recognition and phagocytosis of pathogens and in the induction of response, immunity and immunopathology. A key strategy employed by numerous pathogens such as Yersinia pestis is to circumvent the immune response of the host via actively down-regulating the activation of macrophages. The study on host-pathogen interaction and gene expression is imperative for the development of alternative therapeutics. We have combined Suppression Subtractive Hybridisation (SSH), Microarray techniques, Northern blot analysis and quantitative reverse transcription coupled PCR (RT-PCR) to gain a view of differential host gene expression in response to Y. pestis-26 degrees C infection. In our study, a total of 22 different genes were identified as up-regulated in response to the Y. pestis infection. These genes include unknown EST's, cytokines, enzyme of cytokine, receptors, ligands, transcriptional factors, inhibitor of transcriptional factor, and proteins involved with cytoskeleton. More interestingly, among them are 7 genes that encode for factors known to be associated with cell cycling and cell proliferation, with 3 of them playing a role in apoptosis. Our data also indicate that macrophage cells undergo apoptosis during an infection with Y. pestis-37 degrees C, however an infection with 26 degrees C cultures results in a delayed apoptosis. The correlation between the delayed apoptosis and the up-regulation of anti-apoptotic gene is currently being studied.