Role of granulocytes and monocytes in experimental Staphylococcus epidermidis endocarditis.

The role of granulocytes and monocytes during the induction and course of Staphylococcus epidermidis endocarditis was investigated by the selective depletion of monocytes with the drug VP16-213 and of both 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 X 10(5) CFU in control rabbits and 3.4 X 10(4) CFU in the monocyte-depleted rabbits, whereas no significant differences were found between the latter and those depleted of both granulocytes and monocytes. Also, control rabbits injected with 10(6) or 10(7) 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, whereas this effect was not seen in rabbits whose vegetations became infected. The course of the endocarditis appeared to be significantly influenced by both granulocytes and monocytes. Comparison showed that a decrease of the same numbers of these cells per microliter of blood was accompanied for the monocytes by an approximately fourfold higher increase of the number of staphylococci in the vegetations. The correlation between the number of granulocytes and of monocytes on the one hand and the number of staphylococci in the vegetations on the other was not substantially influenced by the duration of the disease or the number of staphylococci injected to induce the endocarditis. The number injected proved to be significantly correlated with the number of staphylococci in the vegetations. In rabbits with numbers of CFU per gram of vegetation exceeding 10(7), 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 between the number of staphylococci in the vegetations and in the circulation was found. We conclude that only monocytes have a measurable effect on the induction of Staphylococcus epidermidis endocarditis but during its course both granulocytes and monocytes keep the endocardial infection in check.