Defining “Dominance” in Atypical Hemolytic Uremic Syndrome: Distinguishing Amplification From Initiation

To the Editor: We read the article by Santarsiero et al., 1 where the authors concluded that the alternative pathway (AP) is "dominant" in atypical hemolytic uremic syndrome based on endothelial ex-vivo assays.Although the data confirm that AP blockade dampens complement deposition in remission samples, the conclusion of "dominance" is methodologically undercontrolled and obscures the distinction between in vitro biological suppression and clinical efficacy.The pathophysiology of atypical hemolytic uremic syndrome and complement-mediated thrombotic microangiopathy is multifactorial, and no single ex vivo system, whether human microvascular endothelial cells-1, glomerular endothelial cells, or HEK293, can fully recapitulate its biology.Cell-based assays share a limitation-they cannot model the dynamic in vivo microenvironment, including the interplay of shear stress, glycocalyx denudation, inflammatory microparticles, and coagulation activation.These systems are tools to interrogate intrinsic serum stimuli and model disease biology.To claim "dominance" requires a comparative analysis; however, the study design lacks necessary controls to evaluate the classical pathway.For the assays, the authors used buffers permissive of all pathways rather than AP-specific conditions (e.g., Magnesiumethyleneglycol-bis-(β-aminoethylether)-N,N,N ′ ,N ′ -tetraacetic acid) and failed to employ C4d staining or specific classical pathway inhibition (e.g., sutimlimab).Without these controls, the authors cannot distinguish between an isolated AP dysregulation and a strong classical pathway stimulus that merely uses the AP for amplification.Our previous work demonstrated increased C4d deposition on human microvascular endothelial cells-1 that was blocked by C1s inhibition. 2 Although the authors show AP inhibition is effective in remission samples, they do not demonstrate that AP inhibitors block C5b-9 deposition in acute phase samples.To support dominance, efficacy must be demonstrated when complement activation is most pronounced.