Measuring human complement activation by HLA antibodies

The clinical significance of HLA antibodies in transplantation depends on their ability to activate complement, which is measured by the standard complement-dependent cytotoxicity test. Recent reports indicate that C3b measurement on target cells may be a better indicator of human complement activation than standard cytotoxicity.

To determine the characteristics of the test, the role of other complement components, and the potential influence of the patient's own complement activity.

The T-cell deposition of multiple complement components triggered by HLA alloantibodies was evaluated by flow cytometry using normal human serum as the source of complement. Complement activity in patients' sera was measured after activation with a standard antibody.

When HLA antibodies activate complement, C3b deposition is usually highest, followed by that of C4b and C5b. Deposition of C1q, C3d, iC3b, or C5b-9 was low or undetectable in this study. The C5 was activated only when relatively high levels of C3b were present. There was a critical concentration of antibody, unique for each patient, below which activation of C3 declined abruptly. The complement activity in the serum of candidates for liver, heart, and lung transplantation that was tested with a standard antibody was similar to normal serum. In contrast, kidney transplant candidates exhibited higher complement activity. Unexpectedly, serum C3 activity was retained for at least 72 hours after collection.

Measurement of C3b, and in some instances C4b or C5b, offers a better definition of the ability of HLA antibodies to activate human complement than tests that are in current use. The results provide a necessary baseline to conduct clinical correlation studies.