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Actinobacillus pkuropneumoniae produces several hemolysins/cytotoxins that may be important in the pathogenesis of acute lesions. Little is known, however, about the role of these virulence factors in chronic disease or the carrier state. We investigated the effects of live bacterial infection and transthoracic injection of a sterile culture supernatant on primary lymphoid organs and lymphocyte populations. Transthoracic inoculation of mice or intranasal inoculation of pigs with virulent A. pleuropneumoniae serotypes 1 and 7 induced thymic cortical lymphoid necrosis. These lesions were reproduced in mice by transthoracic injection of a concentrated sterile culture supernatant. The cytotoxic effect of this culture supernatant was also demonstrated in vitro by using a tetrazolium dye reduction assay. Both porcine and murine thymic lymphocytes as well as splenic T lymphocytes were susceptible to the toxin. Porcine convalescent serum, but not preimmune serum, prevented thymic lesions and neutralized the in vitro cytotoxic effect of the culture supernatant on murine thymic lymphocytes. Thymic lesions also were reproduced in mice by using purified lipopolysaccharide (LPS) from Escherichia coli O111:B4; however, LPS had no in vitro cytotoxic effect on either porcine or murine thymic lymphocytes. These results suggest that secreted A. pleuropneumoniae toxin(s) is capable of affecting host T-lymphocyte populations and may affect host immune function.