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The complement system is a key component of innate and adaptive immune responses. Complement regulation is critical for prevention and control of disease. We have determined the crystal structure of the complement regulatory enzyme human factor I (fI). FI is in a proteolytically inactive form, demonstrating that it circulates in a zymogen-like state despite being fully processed to the mature sequence. Mapping of functional data from mutants of fI onto the structure suggests that this inactive form is maintained by the noncatalytic heavy-chain allosterically modulating activity of the light chain. Once the ternary complex of fI, a cofactor and a substrate is formed, the allosteric inhibition is released, and fI is oriented for cleavage. In addition to explaining how circulating fI is limited to cleaving only C3b/C4b, our model explains the molecular basis of disease-associated polymorphisms in fI and its cofactors. T he complement system is a major component of innate and adaptive immunity whose efficient regulation is critical for prevention and control of disease (1). This regulation is effected by host cells expressing and recruiting a series of proteins that protect them from complement-mediated destruction. A key step in this self-protection is the proteolytic cleavage of complement C3b (and its homolog C4b) by the serine protease complement factor I (fI) (2) in the presence of additional regulatory proteins termed "cofactors" (3). In the absence of regulation by fI the alternative pathway of complement leads to continuous generation of fluid-phase and cell-surface-deposited C3b by a self-amplification loop: The more C3 is converted to C3b, the more C3 convertases are formed, resulting in the depletion of C3 (4). With healthy levels of functional fI and its cofactors, the irreversible breakdown of C3b to iC3b has three effects: (i) arrest of the assembly of the C3 convertases on hostcell surfaces, thus avoiding inappropriate complement amplification; (ii) prevention of runaway C3 consumption in the fluid phase; and (iii) generation of the C3b cleavage fragments that go on to bind specific complement receptors and are involved in opsonization and triggering of the adaptive immune response (5). The importance of this regulatory mechanism is highlighted by the fact that fI-deficient individuals suffer from recurring infections and that polymorphisms in or near the genes for fI (and its cofactors) can predispose the carriers to diseases such as systemic lupus erythematosus, atypical hemolytic-uremic syndrome (aHUS), membranoproliferative glomerulonephritis, and age-related macular degeneration (for a recent review, see ref. 6).During the last few years, biochemical and structural data for complement regulators and C3/C3b in complex with regulators and inhibitors (7-11) have significantly advanced our understanding of the molecular mechanisms underlying complement regulation (12). To date, however, only low-resolution information about fI has been available (13, 14). FI is synthesized as a single 66-kDa chain an...

The complement system is a group of about 35 soluble and cell-surface proteins which interact to recognize, opsonize and clear or kill invading micro-organisms or altered host cells (e.g. apoptotic or necrotic cells). Complement is a major part of the innate immune system. Recognition proteins such as C1q, MBL (mannan-binding lectin) and ficolins bind to targets via charge or sugar arrays. Binding causes activation of a series of serine protease proenzymes, such as C1r, C1s and MASP2 (MBL-associated serine protease 2), which in turn activate the atypical serine proteases factor B and C2, which then activate the major opsonin of the system, C3. Activated C3 binds covalently to targets, and is recognized by receptors on phagocytic cells. Two of the complement proteases, factors D and I, circulate not as proenzymes, but in activated form, and they have no natural inhibitors; their substrates are transient protein complexes (e.g. C3bB and C3bH) which form during complement activation. Factor B and C2 also have no natural inhibitor; they are active only when proteolytically cleaved and bound in an unstable, short-lived complex with C3b or C4b. C1r, C1s and the MASPs, in contrast, are regulated more conventionally by the natural serpin, C1-inhibitor. Complement proteases in general have very narrow specificity, and low substrate turnover with both natural and synthetic substrates. Excessive activation of complement is inflammatory, and causes tissue damage (e.g. in rheumatoid arthritis, or in ischaemia/reperfusion injury). Substances that regulate complement activation are likely to be useful in the regulation of inflammation. Complement activation might potentially be controlled at many different steps. Much attention has been focused on controlling the formation or activity of the protease complexes C3bBb and C4b2a (containing activated factor B and C2 respectively), as these generate the inflammatory peptides C3a and C5a.

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The factors that allow self-reactive B cells to escape negative selection and become activated remain poorly defined. Using a B-cell receptor-knock-in mouse strain, we identify a pathway by which B-cell selection to nucleolar self-antigens is complement-dependent. Deficiency in complement component C4 led to a breakdown in the elimination of autoreactive B-cell clones at the transitional stage, characterized by a relative increase in their response to a range of stimuli, entrance into follicles and a greater propensity to form self-reactive germinal centers. Using mixed bone marrow chimeras we found that the myeloid compartment was sufficient to restore negative selection in the auto-reactive mice. A model is proposed in which in the absence of complement C4, inappropriate clearance of apoptotic debris promotes chronic activation of myeloid cells, allowing the maturation and activation of self-reactive B-cell clones leading to increased spontaneous formation of germinal centers.