Properties of a cell-bound homogeneous murine IgM anti-dextran--V. Effects of density of cell-bound IgM on immunological activities

Structure and function of several anti-dansyl chimeric antibodies formed by domain interchanges between human IgM and mouse IgG2b

Two pairs of chimeric, domain-switched immunoglobulins with identical murine, anti-dansyl (5-dimethylaminonaphthalene-1-sulfonyl) variable domains have been generated, employing as parent antibodies a human IgM and a mouse IgG2b. The first pair of chimeric antibodies mu mu gamma mu and gamma gamma mu gamma was generated by switching the C mu 3 and C gamma 2 domains between IgM and IgG2b. The second pair of chimeras mu mu gamma gamma and gamma gamma mu mu were formed by switching both C mu 3 and C mu 4 with C gamma 2 and C gamma 3. SDS-polyacrylamide gel electrophoresis and analytical ultracentrifugation showed that over half (57 and 71%) of the two chimeric antibodies possessing the C mu 4 domain and tail piece formed disulfide-linked IgM-like polymers. In contrast, the two chimeric antibodies lacking the C mu 4 domain were almost entirely monomeric. Both monomeric chimeras had reduced ability to activate complement. The chimera gamma gamma mu gamma had no activity under any of the assay conditions, whereas mu mu gamma gamma caused only a small amount of cell lysis but was fully active in consuming complement at 4 degrees C. The polymeric chimera gamma gamma mu mu was much less active than IgM, bound C1 weakly and caused some cell lysis but consumed little complement with soluble antigen. The polymeric chimera mu mu gamma mu bound C1 strongly and was the most active antibody in all assays, even more active than the parental IgG2b and IgM antibodies; it was the only antibody that exhibited antigen-independent activity. The results suggest that C mu 3 alone does not constitute the complement binding site in IgM but requires both C mu 1-2 and C mu 4 for full activity.

Cold target competition analysis of the classical activation pathway of complement-mediated cytotoxicity: a non-interaction model for competing lysis

A mathematical analysis of cold target competition experiments of complement-mediated lysis is presented, aimed at developing a minimal model of lysis where no interaction between the competing populations of sensitized blood group A and B erythrocytes is presumed. The model is able to predict the extent of lysis from the input values with remarkable accuracy suggesting that under the conditions used no stimulation and/or inhibition of the lysis of the sensitized erythrocytes occurs. The distribution of complement between the competing A and B erythrocyte populations is approximated by the model and found to be proportional to the 5th and 4th power of the ratios of the antibody and target cell concentrations, respectively. In accordance with earlier observations, suggesting that the interaction between the antibody and the C1q molecules is based on polar electrostatic charges, we propose that the sensitizing antibody provides an electrostatic field around the erythrocytes which attracts C1q molecules towards their membranes.

Activation of complement by different immunoglobulin heavy chain isotypes of the channel catfish (Ictalurus punctatus)

The approximately 750,000 mol. wt tetrameric Ab population from catfish immunized with sheep erythrocytes (E) was purified and shown to sensitize E to the hemolytic activity of catfish serum complement. Catfish complement was heat-labile at 45 degrees C and was hemolytically inactivated by both EGTA and EDTA. Hemolytic activity of EDTA treated serum was restored by the addition of both Ca+ and Mg2+ ions. In comparative assays, the CH50 titers of catfish sera were similar to the CH50 titers of human sera. Catfish complement was hemolytically active at incubation temps ranging from 3 to 40 degrees C, which suggests that complement should be functional throughout the catfish's normal environmental temps. Human complement exhibited considerable cross-reactivity when assayed on catfish Ab sensitized E at 25 degrees C; however, at 3 degrees C, no hemolytic activity was observed. They findings suggest that there may be structural differences between the complement component(s) of these two systems. Mouse mAbs to the recently described H chain isotypes of catfish Ig were used to fractionate catfish anti-sheep E antisera. Each of the purified isotypes contained Ab which sensitized E to the hemolytic activity of catfish complement. Dose-response analysis of the average number of hemolytic sites per cell, as a function of relative Ab concn, predicts that the catfish tetramer conforms to the one-hit theory of immune hemolysis. Purified catfish anti-fluorescein Ab of each isotype was also reacted with fluorescein-labeled sheep E and the level of complement-mediated hemolysis determined. Dose-response analysis indicated that similar numbers of Ab molecules bound to haptenated E resulted in similar levels of complement-mediated lysis. Lastly, the effect of cell surface hapten density of target cells was examined. These studies, similar to those reported with human IgM, showed that the binding of the catfish tetramer to a cell surface hapten may not be sufficient to activate lytic complement.

The time development of direct hemolytic plaques: implications for the binding of IgM to cell surface haptens

We studied the time development of direct hemolytic plaques in thin layers containing either sheep red blood cells (SRBC) directly haptenated (DH) with trinitrophenyl, or SRBC indirectly haptenated (IH) with dinitrophenyl coupled to human serum albumin. The DH-SRBC tend to be sparsely haptenated while the IH-SRBC tend to have very high local hapten densities. We observed marked differences in the growth of plaques for the two differently haptenated SRBC. Plots of the plaque radius squared vs time show that the slope of those curves that developed in a lawn of DH-SRBC tended to be constant while the slope of those curves that developed in a lawn of IH-SRBC tended to decrease with time. These results are what is predicted from theory if: IgM binds to DH-SRBC through attachments that rapidly dissociate, if IgM binds to IH-SRBC through attachments that very slowly dissociate, and if (3) both types of bound IgM can fix and activate complement.

Mouse monoclonal antibodies at the red cell surface--II. Effect of hapten density on complement fixation and activation

Complement (C)-dependent hemolytic dose-response curves of anti-TNP IgG2b and IgM monoclonal antibodies as a function of TNP density were analyzed: sheep red cells coupled with TNP served as targets. Under conditions when equal numbers of either IgG2b or IgM anti-TNP antibodies were taken up by cells with various TNP densities, both antibodies showed optimal activity at a hapten density of approximately 10(6) TNP/E with regard to C-mediated lysis. These results were similar to those obtained with polyclonal antibodies. The effectiveness of monoclonal antibodies in utilizing guinea pig or mouse C was also investigated. IgM anti-TNP monoclonal antibodies lysed E-TNP in the presence of guinea pig C, but failed to produce lysis in the presence of mouse C. Two monoclonal IgG1 (anti-TNP and anti-SRBC) and an IgG2a anti-TNP antibody failed to produce hemolysis in the presence of guinea pig and mouse C. IgG2b monoclonal antibodies, whether directed against TNP or Forssman antigen, activated guinea pig and, to a lesser extent, mouse C. Finally, monoclonal anti-TNP IgG3 antibodies exhibited a low but measurable hemolytic activity with guinea pig C (10-20 times below that of IgG2b).

The limited fragmentation of porcine haemolytic immunoglobulin M. Evidence that haemolytic activity depends on the number of Fab arms that can attach to the erythrocyte surface

Methods previously developed in this laboratory for the proteolytic removal of limited numbers of Fab arms from the IgM molecule have been used to study haemolytic activity. The gradual removal of arms produced a rapid decrease in complement-mediated lysis of erythrocytes but had a much slower effect on their agglutination. Therefore a stage was reached at which molecules with four Fab arms gave good agglutination but did not cause cell lysis. The same result was obtained whether Fab arms were removed randomly by pepsin or in pairs by trypsin. Hence the non-lytic nature of these molecules was independent of the arrangement of their remaining Fab arms. A sharp decrease in haemolytic activity was also obtained under conditions of high antibody concentration when the IgM molecular could attach only a few of its Fab arms to the erythrocyte surface. Possible explanations for the dependence of haemolytic activity on the number of Fab arms discussed.