The effects of a human IgM monoclonal anticardiolipin antibody on pregnancy in a transgenic mouse model

The clinical features of antiphospholipid (or Hughes') syndrome (APS) are most commonly seen in individuals who have raised levels of IgG anticardiolipin antibodies. Most murine models of the syndrome have involved the administration of such antibodies to normal mice. However, APS can occur in the presence of raised levels of serum IgM anticardiolipin antibodies alone. The present study was designed to see if an IgM monoclonal antibody can induce changes in mice similar to those seen in human APS. This antibody, BH1, has previously been derived from a patient with primary APS. In its ligand-binding and idiotypic characteristics it is representative of antiphospholipid antibodies (aPL) found in the serum of patients with APS. In order to minimise the immune response to human IgM, we used transgenic mice (F15) which express, and are predicted to be tolerant of, human immunoglobulin mu chains. The features of APS may develop more readily in individuals who have an existing autoimmune disorder, such as systemic lupus erythematosus (SLE). We therefore crossed these transgenic mice with New Zealand Black (NZB, SLE-prone) mice, and used the progeny (F15 x NZB/F1) in our experiments. Twenty-four F15 x NZB/F1 mice were given BH1, or a control IgM antibody, (A5566) immediately preceding and then three times during pregnancy. There was a reduction in the mean number of foetuses in animals given BH1 compared with those given A5566 (8.6 vs 11.0; P < 0.05), and a similar reduction in mean total foetal weight per pregnancy (9.05 vs 12.73 g; P < 0.05). Two mice showed a marked reduction in platelet count. No evidence of thrombosis was detected macroscopically or histologically. Our results show a lower incidence of APS-type features compared to previous studies in which mice have been administered aPL. This may be because BH1 is an IgM antibody. Nevertheless, the data support the concept that IgM aPL of particular ligand-binding specificities may have a direct pathogenetic role in certain cases of human APS.