Serum heat-labile opsonins in systemic lupus erythematosus.

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Abstract
To study possible mechanisms responsible for the increased susceptibility to infection of patients with active systemic lupus erythematosus (SLE), a study of the serum heat-labile opsonic capacity (HLOC) in such patients was undertaken. With leukocytes from normal donors, the sera of 12 of 30 patients with active SLE demonstrated decreased HLOC for E. coli 075. The phagocytic activity was partially restored by normal serum, suggesting that decreased HLOC was responsible for the defective phagocytosis. While 8 of 10 patients with active SLE and concomitant infections showed deficient opsonic capacity to E. coli 075, only 4 of 20 such patients without infections showed the defect (P = 0.01). None of 12 patients with inactive disease had deficient opsonic capacity. Similar results were obtained with S. aureus 502A as the test bacterium. In the patients surviving infection, recovery of normal serum opsonic capacity was rapid and usually coincided with an increase of serum complement to normal levels. In three patients with active SLE and infection, the causative microorganisms were isolated and opsonic capacity for these organisms tested with the individual patients' sera. In each case, sera obtained at the onset of the infectious episode had low opsonic capacity when compared with normal sera. Serum C3 proactivator levels were low in 9 of 11 sera with deficient opsonic capacity. However, similar low values were found in other SLE sera with normal HLOC, suggesting that other factors of the opsonic system were also depleted. Addition of the classical complement components C1, C4, C2, C3, and C5 to sera with deficient HLOC failed to restore activity. Addition of pure C3 proactivator also failed to restore activity. However, addition of C3 proactivator together with 50 degrees C-heated normal serum restored activity, indicating that factors active at the early steps of opsonic activation via the alternative pathway of complement were necessary to restore opsonic activity. These findings indicate that in active SLE, a decrease of components of the alternate pathway of complement activation results in an acquired defect of serum HLOC and perhaps other related complement-mediated functions. This defect may be an important factor in the increased susceptibility to infections of patients with active systemic lupus erythematosus.