Identity of the segment of human complement C8 recognized by complement regulatory protein CD59

Area of Science:

• Immunology
• Complement System Biology

Background:

• CD59 antigen is a key regulator of the complement system, preventing cell lysis by inhibiting the membrane attack complex (MAC).
• The inhibitory activity of CD59 is species-specific, with human CD59 being less effective against rabbit complement components.

Purpose of the Study:

• To identify the specific region of the C8 alpha-subunit recognized by CD59.
• To elucidate the molecular basis for the species-selectivity of CD59-mediated MAC inhibition.

Main Methods:

• Expression and purification of recombinant human and rabbit C8 alpha-subunit peptides.
• Binding assays using purified CD59 and recombinant C8 peptides.
• Functional analysis of MAC-mediated hemolysis using chimeric C8 proteins in complement-compromised cells.

Main Results:

• CD59 specifically binds to a peptide encompassing residues 334-385 of the human C8 alpha-subunit, requiring a disulfide bond between Cys345 and Cys369.
• No specific binding was observed with the corresponding rabbit C8 alpha-subunit sequence.
• Functional studies confirmed CD59 recognizes a conformationally sensitive epitope within human C8 alpha (residues 320-415), influenced by flanking sequences and C8 beta, but not C8 gamma.

Conclusions:

• The C8 alpha-subunit segment (residues 334-385) contains a critical epitope for CD59 binding and inhibition.
• Species-selectivity in CD59-mediated MAC inhibition is determined by sequence and conformational differences within this C8 alpha-subunit region.
• Optimal CD59 interaction is modulated by adjacent sequences in C8 alpha and the C8 beta chain.