Mouse complement is famously difficult to assay. Mouse serum rapidly loses its activity and the typical titer of mouse serum is far below the 150-200 CH50 units/mL found with human serum. Mouse strains differ greatly in their complement CH50 titers (Ref 1, 2& 3) and a great many common mouse strains are genetically deficient in C5 (Ref 4 & 5) meaning that they cannot lyse cells even if they have an otherwise fully functional complement system. The exact reasons for low titers in C5 sufficient strains are not entirely clear and they may be different for different mouse strains. However, a common reason is that mouse classical pathway components are not efficiently activated by the standard antibodies bound to EA. At Complement Technology, Inc. we have developed a unique hemolytic reagent (MCAR = Mouse Complement Assay Reagent) that sensitizes sheep erythrocytes to provide improved mouse CH50 titers. For example, commercially available mouse complement exhibited a CH50 of only ~12 units/mL in the assay system described below, however, with MCAR present the CH50 was 217. That is 18-fold higher complement titer with MCAR. This is numerically comparable to normal human serum CH50, but the assays are different. At this time we cannot say how many mouse strains this reagent will work with, but we have used MCAR with three and it worked equally well with all.
Mouse serum is also extremely unstable outside of the mouse. We have observed a rapid loss of activity after thawing (100% loss if left at 4oC overnight). Thus, we advise rapidly thawing NMS and immediately using it in assays or if it is necessary aliquoting and freezing the NMS. It should be thawed rapidly in a water bath, moved immediately to wet ice when thawed, aliquoted and re-frozen as rapidly as possible. Upon use, thaw only when the experimental setup is ready for the NMS sample and keep it on wet ice after thawing.
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