C4b-binding protein and factor H compensate for the loss of membrane-bound complement inhibitors to protect apoptotic cells against excessive complement attack.
Apoptotic cells have been reported to down-regulate membrane-bound complement regulatory proteins (m-C-Reg) and to activate complement. Nonetheless, most apoptotic cells do not undergo complement-mediated lysis. Therefore, we hypothesized that fluid phase complement inhibitors would bind to apoptotic cells and compensate functionally for the loss of m-C-Reg. We observed that m-C-Reg are down-regulated rapidly upon apoptosis but that complement activation follows only after a gap of several hours. Coinciding with, but independent from, complement activation, fluid phase complement inhibitors C4b-binding protein (C4BP) and factor H (fH) bind to the cells. C4BP and fH do not entirely prevent complement activation but strongly limit C3 and C9 deposition. Late apoptotic cells, present in blood of healthy controls and systemic lupus erythematosus patients, are also positive for C4BP and fH. Upon culture, the percentage of late apoptotic cells increases, paralleled by increased C4BP binding. C4BP binds to dead cells mainly via phosphatidylserine, whereas fH binds via multiple interactions with CRP playing no major role for binding of C4BP or fH. In conclusion, during late apoptosis, cells acquire fluid phase complement inhibitors that compensate for the down-regulation of m-C-Reg and protect against excessive complement activation and lysis.
Trouw
Leendert A
LA
Department of Laboratory Medicine, University Hospital Malmö, 5-20502 Malmö, Sweden.
Bengtsson
Anders A
AA
Department of Clinical Science, Rheumatology, Lund University, 5-22184 Lund, Sweden.
Gelderman
Kyra A
KA
Unit for Medical Inflammation Research, Lund University, 5-22184 Lund, Sweden.
Dahlbäck
Björn
B
Department of Laboratory Medicine, University Hospital Malmö, 5-20502 Malmö, Sweden.
Sturfelt
Gunnar
G
Department of Clinical Science, Rheumatology, Lund University, 5-22184 Lund, Sweden.
Blom
Anna M
AM
Department of Laboratory Medicine, University Hospital Malmö, 5-20502 Malmö, Sweden. Electronic address: Anna.Blom@med.lu.se.
D017209:Apoptosis / Q000502:physiology*; D002462:Cell Membrane / Q000378:metabolism*; D003167:Complement Activation / Q000502:physiology*; D003176:Complement C3 / Q000378:metabolism; D050716:Complement C4b-Binding Protein / Q000378:metabolism*; D003186:Complement C9 / Q000378:metabolism; D017242:Complement Factor H / Q000378:metabolism*; D015536:Down-Regulation / Q000502:physiology; D006801:Humans; D019169:Jurkat Cells; D008180:Lupus Erythematosus, Systemic / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism*; D011485:Protein Binding / Q000502:physiology
D003176:Complement C3; D050716:Complement C4b-Binding Protein; D003186:Complement C9; D008565:Membrane Proteins; D017242:Complement Factor H