Role of granulocytes and monocytes in experimental Staphylococcus epidermidis endocarditis

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Abstract
The role of granulocytes and monocytes during the induction and course of S. epidermidis endocarditis was investigated by the selective depletion of monocytes with the drug VP16-213 [vepeside] and of granulocytes and monocytes with nitrogen mustard. The induction of endocarditis was influenced only by the depletion of monocytes: the 50% infective dose differed significantly, being 3.4 .times. 105 CFU [colony-forming units] in control rabbits and 3.4 .times. 104 CFU in the monocyte-depleted rabbits; no significant differences were found between the latter and those depleted of granulocytes and monocytes. Control rabbits injected with 106 or 107 CFU had a significantly higher incidence of sterile vegetations than did rabbits selectively depleted of granulocytes or monocytes. Compared with baseline values, mean monocyte numbers at the time of bacterial inoculation were significantly increased in control rabbits whose vegetations remained sterile; this effect was not seen in rabbits whose vegetations became infected. The course of the endocarditis appeared to be significantly influenced by granulocytes and monocytes. Comparison showed that a decrease of the same numbers of these cells per .mu.l of blood was accompanied by an .apprx. 4-fold higher increase of the number of staphylococci in the vegetations. The correlation between the number of granulocytes and of monocytes, and the number of staphylococci in the vegetations was not substantially influenced by the duration of the disease or the number of staphylococci injected to induce the endocarditis. The number injected was significantly correlated with the number of staphylococci in the vegetations. In rabbits with vegetations containing > 107 CFU/g, blood cultures were usually positive. This finding applied rarely to control rabbits, but generally to drug-treated rabbits. In the latter animals, a significant correlation was found between the number of staphylococci in the vegetations and in the circulation. Thus, only monocytes have a measurable effect on the induction of S. epidermidis endocarditis; during its course, granulocytes and monocytes keep the endocardial infection in check.