THE DTH TEST
To date, no in vivo measures of immune response to the DC-based vaccines are available, except for delayed-type hypersensitivity (DTH) skin tests,
which are not commonly used. Reluctance to incorporate the DTH skin test into vaccine monitoring is related to the necessity
for measurements of skin induration, which must be performed 48–72 hours after antigen application, and requires the patient's
return to the clinic. It is possible to measure DTH to recall antigens (PPD, tetanus, candida) or to specific epitopes used
in a vaccine, or both. As such, the DTH skin test represents a useful strategy for assessing the general status of recall
immunity and for determining the likelihood of mounting a response to the vaccine. It has been reported that a good qualitative
relationship exists between DTH responses and ex vivo T-cell responses to peptides used for vaccination in patients with cancer.40 Most important, a conversion from a negative to positive skin test to the vaccine is an excellent sign of vaccine-induced
immunity, and it has been reported to correlate with clinical responses in patients with melanoma. For these reasons, consideration
should be given to a more frequent use of DTH tests measuring responses to vaccine components and administered before and
after vaccination. A positive DTH skin test to the vaccine after its administration provides a strong rationale for immune
monitoring, using peripheral blood or tumor-draining lymph nodes, if the latter are available. Previously, we have described
the strategy depending on DTH skin test results for selecting monitoring assays.41
Use of DTH to Evaluate Anti-tumor Immunity
When designing immune monitoring for DC-based vaccination trials, it is necessary to measure vaccine-specific responses, although
responses to PMA/ionomycin, anti-CD3 antibody, or mitogens measured in parallel could be used as a general guide for immune
responsiveness of an individual patient. There are two reasonable initial approaches: measure antibody responses to the vaccine,
and look for a shift from IgM to IgG responses in serial serum specimens; or, measure proliferative responses of lymphocytes
(or cytokine production) to the vaccine in bulk cultures. Both can be considered as first-line screening assays. Pre-vaccine
and post-vaccine specimens should be tested together in the same assays.
If both assays give negative results, but a DTH skin test is positive, further ex vivo testing is required, because it may be possible that the bulk assays are not sensitive enough to detect antigen- or peptide-specific
responses. The decision must be made whether to proceed with single-cell assays such as ELISPOT, cytokine flow cytomery (CFC),
or tetramer assay(s) without in vitro sensitization (IVS) or to resort to single-cell assays performed after IVS. This is a difficult decision because if the frequency
of antigen- or peptide-specific T cells is low, then direct (no IVS) single-cell assays could be negative. On the other hand,
the IVS strategy allows for in vitro expansion of antigen- or peptide-specific T cells before the assay, but it is time-consuming, labor-intensive, and costly.
If both screening assays give positive results, consider single-cell assays without IVS for determining the frequency of antigen-
or peptide-responsive T cells. A lack of systemic in vivo response to antigens (peptides) used in the vaccine suggests that ex vivo assays also will be negative and that the patient is unlikely to respond immunologically to the vaccine. Nevertheless, immune
monitoring with the most sensitive methods is necessary to formally document the absence of cellular immunologic responses.
In such circumstances, the availability of a standardized, robust, single-cell assay that measures function and cellular phenotype
is important.
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