Immunofluorescence Microscopy

Detection of C4d has proved to be useful in the detection of humoral rejection (Fig. 20.8). After antibody binds to antigen, C4 is proteolytically cleaved by activated C1 into C4a and C4b. The cleavage of C4 exposes the reactive and short-lived thiol group in C4b that binds covalently to nearby molecules containing amino or hydroxyl groups, such as proteins and carbohydrates (40,41). Bound C4b is proteolytically inactivated into

Figure 20.8. Acute humoral rejection. C4d is present in peritubular capillaries (immunofluorescence on frozen section; monoclonal antibody to C4d).

C4d, a 44.5-kd peptide, which contains the thioester site and remains covalently bound at the same site. Thus, if C4d is bound to a structural protein, it is potentially a durable marker of local complement activation by the classical pathway. C4d can disappear as soon as 8 days (28), and after 60 days is usually negative (unless chronic humoral rejection develops) (19). Cases with fibrinoid necrosis of arteries typically contain immunoglobulin (Ig; usually IgG and IgM), C3, and fibrin in the arterial walls. The criteria for acute humoral rejection proposed by Mauiyyedi et al (19) and accepted with minor modification by the Banff conference are given in Table 20.3 (39):

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