**5.2. Positive T-cell CDC crossmatch**

In the situation of a high titre of high avidity DSA it may be that many dilutions are required for the test to become negative (e.g. 1 in 128). With antibody at a low level or one with a low affinity, a single dilution may be enough to render the crossmatch result negative. This may also give an indication as to the likelihood that a negative crossmatch could be achieved with

The basic CDC crossmatch can be enhanced by the addition of antihuman globulin (AHG). This technique increases the sensitivity of the CDC crossmatch as a result of multiple AHG molecules binding to each DSA attached to thedonor cells thereby amplifying the total number of Fc receptors available for interaction with complement component 1, which increases the

It is also possible to have a negative crossmatch in the presence of a DSA and this can happen

**3.** antigen for which the antibody is specific is expressed only at very low levels on the

A further consideration relates to variations in antibody levels in a given individual's serum samples, collected at different times. The most reactive serum is generally called the 'peak serum'. This may have been collected several years earlier, with the 'current serum' showing quite different reactivity. As an example, the peak serum may show a clear positive CDC crossmatch result, but as the antibody levels have fallen in subsequent sera, so too may the degree of cell lysis in the assay. This may render the CDC crossmatch negative. Nevertheless, the antibodies found in the peak sera may still be of relevance, indicating that re-exposure to the relevant antigen could initiate a memory response with the risk of early and aggressive rejection. For this reason, patients on transplant waiting lists have sera collected at frequent intervals; variations can be monitored and newly appearing HLA antibodies can be detected.

There are important differences in HLA expression between T and B cells, which influence the interpretation of the crossmatch. T cells do not constitutively express HLA class II so the result of a T-cell crossmatch generally reflects antibodies to HLA class I only. B cells on the other hand express both HLA class I and II, as well as a larger range of surface markers, icluding Fc receptors. Because of this, a positive B-cell crossmatch is more difficult to interpret than a positive T-cell cross match. It may be due to antibodies directed against HLA class I or II or both, or it may be due to antibody binding to other sites, that may or may not be clinically important. Hence, if the T- and B-cell crossmatches are positive the interpretation is that there may be either single or multiple HLA class I DSA/s or a mixture of HLA class I and II DSA. While a negative T-cell crossmatch in the setting of a positive B-cell crossmatch suggests either there may be one or more class II DSA/s but no class I antibodies or that there is a low-level DSA to a class I antigen with greater lysis of B cells relative to T cells. This is often due to the fact that B cells express higher levels of HLA class I than do T cells [26]. When class I comple‐ ment fixing HLA DSA are present at a significant level one would expect both the T and B-cell

a desensitization protocol.

in the following conditions:

donor's lymphocytes.

likelihood of complement activation and cell lysis.

382 Current Issues and Future Direction in Kidney Transplantation

**1.** antibody titre is too low to cause complement activation

**2.** antibody is of a type that does not activate complement and

Transplanting in the setting of a positive T-cell crossmatch, which is not due to an autoanti‐ body, is likely to generate a very poor outcome. Patel and Terasaki described the outcomes of 30 such transplants [17]. Twenty four (24) patients lost their grafts immediately to HAR while another three lost their grafts within 3 months. It is not clear why the other three patients had less severe reactions but it may relate to false positive crossmatches generated by autoanti‐ bodies given that dithiothreitol (DTT) which cleaves the multimers of IgM antibodies was not used in their assays. Other possibilities include false positive tests or lower immunogenicity of the antibodies or antigens in those cases.

A recent study investigated whether IVIg or plasma exchange was more effective at desensi‐ tizing crossmatchpositive recipients so that they might be crossmatch-negative at the time of transplant [27]. While most patients were successfully desensitized there was a group of 10 patients who did not achieve a negative crossmatch but were still transplanted. Of this group 70% developed AMR with 50% losing their grafts. Given this data, even after reducing the antibody titre with a desensitization protocol before transplant, a persistent positive T-cell crossmatch remains an absolute contraindication to transplantation.
