Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease affecting multiple organs. Using a recently developed flow cytometric assay, we discovered the deposition of significant amounts of complement C4-derived activation product on the surface of erythrocytes of SLE patients. Additional interesting findings emerging from the preliminary studies include: 1) elevated and fluctuating levels of erythrocyte-bound C4-derived activation products (E-C4) were detected among SLE patients, and 2) distinct subpopulations of erythrocytes with different E-C4 levels were concurrently present in the same patient. These findings have led to the central hypothesis of this proposal that erythrocytes and reticulocytes may serve as, respectively, "time capsules" and "instant messenger" of the inflammatory condition in vivo (and thus disease activity) in SLE patients. On one hand, erythrocytes circulating during a disease flare may acquire an increased amount of C4-derived activation products on their surface, whereas erythrocytes emerging from the bone marrow during remission may bear decreased levels of C4-derived products. On the other hand, reticulocytes (the youngest form of erythrocytes), when emerging from the bone marrow during an active disease state, may immediately be exposed to and acquire high levels of C4-derived products. Therefore, determination of the levels of C4-derived activation products on the surface of erythrocytes and reticulocytes circulating at any given time should reveal disease activity during the preceding 120 days (the life span of erythrocytes) and may provide clues to ongoing/forthcoming disease activation. The research proposed in this application is aimed to verify this hypothesis (Specifics Aim 1 and 2), to investigate the relationship between C4 polymorphism and the deposition of C4-derived products on erythrocytes/reticulocytes (Specific Aim 3), and to elucidate the biochemical basis of the deposition of C4-derived products on erythrocytes/reticulocytes (Specific Aim 4). The proposed studies will be accomplished by i) flow cytometric analysis of age-fractionated erythrocytes and reticulocytes, ii) statistical analysis of the correlation between E-C4 levels, reticulocyte-bound C4 levels, and SLE disease activity, iii) genotyping and phenotyping of C4 in SLE patients and healthy controls, and iv) biochemical studies of the interaction between erythrocytes and C4. These proposed studies will constitute the first endeavor aimed to investigate the biochemical and clinical relevance of the interaction between complement and erythrocytes/reticulocytes, in the context of SLE disease activity. Ultimately, information derived from the proposed studies may aid in the development of new laboratory tests for earlier and more accurate detection of SLE flares, and thereby facilitating the formulation and assessment of new therapeutic approaches for SLE.