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The placement of a ligature around the second maxillary molar of the conventional rat caused osteoclastic bone resorption and simultaneously, alveolar bone formation. The number of peripheral mononuclear cells in the blood and lymphoblastic transformation of spleen cells in response to concanavalin A increased. Cyclophosphamide (CY), an immunosuppressive agent, given shortly after placing the ligature suppressed the lymphoid reactions, spleen size, and bone formation and enhanced bone destruction. CY given in higher doses also suppressed the number of PMN cells. Septicemia developed in several of these animals. Pseudomonas aeruginosa, Klebsiella pneumoniae and Escherichia coli were isolated from the blood and/or ligature. Antibiotics prevented bone destruction. Without placing a ligature, the high dose of CY did not result in bone loss. These findings suggest that 1) bone destruction of the ligature‐treated rat is of bacterial origin, 2) CY suppresses proliferation of osteoblasts but does not seem to interfere with the activity of osteoclasts, and 3) suppression of the host defenses greatly facilitates bone destruction.

Oral isolates of Actinobacillus actinomycetemcomitans (strain Y4) release spherical microvesicles in large numbers during normal growth. The biological activities of these products were studied, and it was estimated that approximately 1/10 of their dry weight was made up of heat-and proteolysis-resistant endotoxin. The chicken embryo lethality and bone-resorbing activity of the microvesicles were heat stable but proteolysis sensitive. Other laboratories have reported the presence of a heat-and proteolysis-sensitive leukotoxin in similar preparations. Accordingly, the microvesicles released by strain Y4 may contain, in addition to endotoxin, several potent substances which are highly toxic and active in bone resorption, and these may be significant factors in the pathogenesis of periodontal diseases.

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