Nuclear imaging and the assessment of human Fc receptor function: studies in systemic lupus erythematosus

In vivo immune clearance of immunoglobulin G sensitized autologous erythrocytes (rbc) was studied in nine patients who fulfilled American Rheumatism Association (ARA) criteria for systemic lupus erythematosus (SLE). Opsonized rbc were radiolabelled with technetium 99. The rate of radioactive blood clearance was measured, as was organ-specific radioactive uptake utilizing area-of-interest (AOI) measurements in a computerized scintigraphic imaging system. It was shown that dynamic quantitation of an AOI corresponding to the heart generated time-activity curves which approximated blood-clearance curves. Calculation of first order clearance (min per 50% decrease in counts) showed a highly significant correlation between rates derived from heart AOI curves and those derived from blood-clearance curves (r = 0.9483, P = 0.0003). Clearance curves showed a monoexponential slope in most patients. Further exploration of organ-specific AOI curves showed that percent splenic uptake and the nature of the splenic uptake curves varied between patients. These studies point toward a variable splenic role in Fc receptor function for SLE patients and further demonstrate the utility of nuclear imaging in studying immune clearance.