Question

1a. What is CR3? What is the normal ligand for CR3, and what is the consequence when CR3 is bound by this ligand?

While the 2 antibodies used both bind extracellular domains on CR3, they do not bind to the exact same part of the CR3 molecule. The M1/70 antibody (Ab) binds the same portion of CR3 as does the normal ligand (which you answered in 1a). The 5C6 antibody binds a separate extracellular domain. When these antibodies are used in the research setting, they have the effect of blocking CR3 from binding other ligands; in other words, they are used as reagents to block ligand-receptor binding.

            Use CGD neutrophil killing assay from Holmes et al (1966). How might the outcome of the experiment been different if, in addition to pre-incubating the S. aureus with serum, they also pre-incubated the neutrophils with antibody M1/70, and then combined the bacteria and neutrophils? Use a sketch if it helps you think through the problem, and explain your answer.

Answer 1

CR3 is a reseptor activated on phagocytosis cells, b cells and natural killer cells. Being an intigrin family protein as well as receptor it helps in adhesion, phagocytosis and complement reactions. It is a membrane receptor, mediating recognition of diverse ligands such as intercellular adhesion molecule-1 (ICAM-1) and fixed iC3b.

In the first case where the cells are preincubated with serum containing antibodies then the Cr3 receptor binds to M1 / 70 antibody where it's ligand supposed to bind, but in the second case where the cells are pre-incubated with bacteria and antibody, then the compilation of bacteria and antI bodies for the receptor, hence some receptors have bacteria on them and some have M1/70 attached.