The complement system is an essential component of the innate immune system that participates in elimination of pathogens and altered host cells and comprises an essential link between the innate and adaptive immune system. Soluble and membrane-bound complement regulators protect cells and tissues from unintended complement-mediated injury. Complement factor H is a soluble complement regulator essential for controlling the alternative pathway in blood and on cell surfaces. Normal recognition of self cell markers (i.e. polyanions) and C3b/C3d fragments is necessary for factor H function. Inadequate recognition of host cell surfaces by factor H due to mutations and polymorphisms have been associated with complement-mediated tissue damage and disease. On the other hand, unwanted recognition of pathogens and altered self cells (i.e. cancer) by factor H is used as an immune evasion strategy. This review will focus on the current knowledge related to these versatile recognition properties of factor H.

Factor H (fH) is essential for complement homeostasis in fluid-phase and on surfaces. Its two C-terminal domains (CCP 19-20) anchor fH to self surfaces where it prevents C3b amplification in a process requiring its N-terminal four domains. In atypical hemolytic uremic syndrome (aHUS), mutations clustering towards the C-terminus of fH may disrupt interactions with surface-associated C3b or polyanions and thereby diminish the ability of fH to regulate complement. To test this we compared a recombinant protein encompassing CCP 19-20 with sixteen mutants. The mutations had only very limited and localized effects on protein structure. While we found four aHUS-linked fH mutations that decreased binding to C3b and/or to heparin (a model compound for cell-surface polyanionic carbohydrates), we identified five aHUS-associated mutants with increased affinity for either or both ligands. Strikingly, these variable affinities for the individual ligands did not correlate with the extent to which all the aHUS-associated mutants were found to be impaired in a more physiological assay that measured their ability to inhibit cell surface complement functions of full-length fH. Taken together, our data suggest that disruption of a complex fH-self surface recognition process, involving a balance of affinities for protein and physiological carbohydrate ligands, predisposes to aHUS.

Uncontrolled activation of the alternative pathway of complement is thought to be associated with age-related macular degeneration (AMD). The alternative pathway is continuously activated in the fluid phase, and tissue surfaces require continuous complement inhibition to prevent spontaneous autologous tissue injury. Here, we examined the effects of oxidative stress on the ability of immortalized human retinal pigment epithelial cells (ARPE-19) to regulate complement activation on their cell surface. Combined treatment with H 2 O 2 (to induce oxidative stress) and complementsufficient serum was found to disrupt the barrier function of stable ARPE-19 monolayers as determined by transepithelial resistance (TER) measurements. Neither treatment alone had any effect. TER reduction was correlated with increased cell surface deposition of C3, and could be prevented by using C7-depleted serum, an essential component of the terminal complement pathway. Treatment with H 2 O 2 reduced surface expression of the complement inhibitors DAF, CD55, and CD59, and impaired regulation at the cell surface by factor H present within the serum. Combined treatment of the monolayers with H 2 O 2 and serum elicited polarized secretion of vascular epidermal growth factor (VEGF). Both, secretion of VEGF and TER reduction could be attenuated using either an alternative pathway inhibitor or by blocking VEGF receptor-1/2 signaling. Regarded together, these studies demonstrate that oxidative stress reduces regulation of complement on the surface of ARPE-19 cells, increasing complement activation. This sublytic activation results in VEGF release, which mediates disruption of the cell monolayer. These findings link oxidative stress, complement activation, and apical VEGF release, which have all been associated with the pathogenesis of AMD.

The high resistance of Trypanosoma cruzi trypomastigotes, the causal agent of Chagas’ disease, to complement involves several parasite strategies. In these in vitro studies, we show that T. cruzi calreticulin (TcCRT) and two subfragments thereof (TcCRT S and TcCRT R domains) bind specifically to recognition subcomponents of the classical and lectin activation pathways (i.e., to collagenous tails of C1q and to mannan-binding lectin) of the human complement system. As a consequence of this binding, specific functional inhibition of the classical pathway and impaired mannan-binding lectin to mannose were observed. By flow cytometry, TcCRT was detected on the surface of viable trypomastigotes and, by confocal microscopy, colocalization of human C1q with surface TcCRT of infective trypomastigotes was visualized. Taken together, these findings imply that TcCRT may be a critical factor contributing to the ability of trypomastigotes to interfere at the earliest stages of complement activation.

A common single nucleotide polymorphism in the factor H gene predisposes to age-related macular degeneration. Factor H blocks the alternative pathway of complement on self-surfaces bearing specific polyanions, including the glycosaminoglycan chains of proteoglycans. Factor H also binds C-reactive protein, potentially contributing to noninflammatory apoptotic processes. The at risk sequence contains His Age-related macular degeneration (AMD) 2 is a leading cause of irreversible visual impairment in the elderly (1). The densely packed photoreceptors in the macula are maintained by the underlying retinal pigment epithelium (RPE) (2). A unique pentalaminar extracellular matrix (ECM), the Brüch's membrane, separates the RPE from the fenestrated endothelium of the choroidal vasculature. In early AMD, fatty deposits, called drusen, appear between Brüch's membrane and the RPE (3). Early AMD may progress to severe forms (4), characterized by RPEcell death and atrophy of the photoreceptor layer, or choroidal neovascularization.Drusen are rich in cell breakdown products and proteins of the complement system (5). Complement (6, 7) is a potent mediator of inflammation. An association between AMD and the gene (CFH) for the complement regulator factor H (fH) was demonstrated by Hageman et al. (8) and confirmed independently by others (9 -12). Hageman et al. (8) additionally reported the RPE to be a source of fH, while drusen components C3a and C5a have been implicated in neovascularization in mouse models of AMD (13), strengthening further the evidence for an AMD-complement link.The at-risk allele that has received most attention, in CFH exon 9 (rs1061170; 1277T Ͼ C), encodes a His rather than a Tyr at position 402 (residue 384 of the mature fH) and is present in ϳ35% of individuals of European descent. Homozygous individuals have a 6-fold increased risk of developing AMD, whereas heterozygotes are 2.5 times more susceptible (14). Doubts over a causal link between the H402Y variation and the etiology of AMD have been raised by the identification of 20 synonymous or intronic single nucleotide polymorphisms (SNPs) in a 123-kb region overlapping CFH that are even more strongly associated with AMD (15, 16). Moreover, variations in genes encoding complement factor 2 (C2) and complement factor B (BF) also strongly influence risk, as does an SNP in a gene, LOC387715, of unknown function (16,17). Other risk factors