Studies on the Seventh Component (C′7) of Complement

From experiments with sucrose density gradient ultracentrifugation and Sephadex G-200 gel filtration the molecular weights of guinea pig C′7, human C′7 and rabbit C′7 were estimated to be 120,000, 130,000 and 130,000, respectively. In zone electrophoresis on Geon or agarose, C′7 of human, guinea pig and rabbit serum migrated, in this order, in the β globulin region.

The C′7 of the three species reacted well with EAC′1–6 prepared with guinea pig components. From quantitative analysis of C′7 site formation, the dose response curve yielded a straight line when this reaction step was analyzed with the aid of Mayer's one-hit theory. Guinea pig C′7 reacted with EAC′1–6 at an ionic strength of µ = 0.15 as well as at an ionic strength of µ = 0.74. C′7 activity was shown to disappear from the fluid phase after the reaction with EAC′1–6.

Guinea pig antiserum to rabbit C′7 inhibited the hemolytic activity of human and rabbit C′7 but not that of guinea pig C′7.

Separation and Function of the Fourth Component of Complement

Various methods of purification of C′4 have been reported; however, hemolytic activity of C′4 was occasionally lost during purification and it was difficult to separate C′4 from the C′1 inactivator. The following new method of purification of C′4 was then developed: By salting out serum with sodium sulfate, C′4 activity was separated from the C′1 inactivator; loss of C′4 activity was prevented by blocking the action of C′4 inactivating substances with di-isopropyl fluorophosphate. Further purification was achieved by chromatography on carboxymethyl (CM)- and diethylaminoethyl (DEAE)-cellulose columns, and by gel filtration on Sephadex G-200.

Guinea pig, human and rat C′4 were separated by this new process and the effects of C′4 sites on the formation of C′3 sites and C′5 sites were investigated.

Human C′3 was more reactive with EAC′1gp4hu2gp than with EAC′1gp4gp2gp as detected by immune adherence; guinea pig C′3 was equally reactive with the two kinds of cells.

Studies on the Mechanism of Immune Adherence

Sheep erythrocytes sensitized with antibody and carrying on their surface the first four components of complement (EAC′1423) are capable of adhering to human erythrocytes. Removal of C′1 and C′2 from EAC′1423 does not affect the immune adherence reaction (IA).

Treatment of EAC′43 or EAC′4 with pronase results in the removal of C′4 without affecting the antibody. Pronase-treated EAC′43 cells retain IA activity with human erythrocytes.

Tannic acid-treated cells can be coated with highly purified C′3. The resulting C′3-coated sheep erythrocytes adhered to human erythrocytes but not to non-primate erythrocytes. Treatment of C′3-coated cells with C′3 inactivator destroys their ability to adhere to human erythrocytes. Isotope-labeled C′3 on tanned cells was 1/32 as effective in IA as C′3 on EAC′1423.

These results indicate that C′3 is the only component required for the IA reaction with human erythrocytes, and may enable the elucidation of the molecular structure of C′3 in relation to its immune adherence reactivity.