In the last decade, CTAs have been recognized as promising targets for immunotherapy. The interest in CTAs is based primarily on their almost exclusive tumor-associated pattern of expression. Their presence in normal adult tissues is restricted to male germ cells, although they show variable expression in several types of cancer. In solid cancers, such as carcinomas of the head and neck, non-small cell lung cancers, and melanoma, CTA expression is prevalent, although other types of malignancies such as renal cell and colorectal carcinomas show very low CTA expression.11
Most hematologic malignancies appear to have a low level of CTA expression, yet there are exceptions. Several studies indicated that myelomatous plasma cells highly express several CTAs at both the molecular and protein level.15, 16, 17, 18
Most interestingly, expression of CTAs in clonal plasma cells parallels the activity of disease.15, 16
Less CTA expression was found in monoclonal gammopathy of undetermined significance, whereas increased expression was found in early-stage symptomatic myeloma, and even more so in advanced disease at 13%, 75% and 82%, respectively.16
In addition to the actual presence of CTA in multiple myeloma, spontaneous humoral responses against CTAs have been documented. In a previous study of 46 patients with multiple myeloma, antibodies to CTAs NY-ESO-1, CT7, CT10 and SSX4 were found. In another analysis of 67 newly diagnosed multiple myeloma patients, antibody response to NY-ESO1 was associated with inferior overall survival independent of other prognostic factors.29
Consequently, CTAs are currently under investigation as potential vaccine targets in myeloma.30
The pathologic plasma cell in AL amyloidosis is thought to be similar to the plasma cell seen in multiple myeloma, with common molecular defects.31
We found that CTAs are, in fact, present in AL amyloidosis. The most prevalent CTA is CT7/MAGE-C1, which we identified in 25/38 (66%) cases. The expression pattern was predominantly focal, i.e., <5% of tumor cells were immunopositive. A high distribution of CT7 expression, i.e., in >50% of the plasma cells, was seen in only two cases. In those cases, there was no difference in percent plasma cell burden with any CT7 expression compared with those with none. However, those patients with >5% CT7 expression had a higher percent plasma cell burden compared with those with just focal expression by IHC. The high prevalence of CT7/MAGE-C1 expression in our AL series shares similarities with previous studies in myeloma, where incidences of >80% positivity were observed.15, 17
However, although the expression in myeloma expression usually encompasses a majority of plasma cells, immunopositivity is generally restricted to only few plasma cells in amyloidosis.
There was a markedly low incidence of CTA expression other than CT7 in our series. CT10 and GAGE were expressed in only 3/38 (7%) and 1/38 (3%) cases, respectively. For both antigens, only single cells were immunopositive. There was no expression for any of the other CTAs tested. Conversely, in myeloma, a much higher incidence of positivity was reported for several CTAs that were analyzed in our study, such as CT10/MAGE-C2 (>50%), NY-ESO-1 (>20%) and MAGE-A1 (>20%).15, 17
Taken together at a protein level, AL amyloidosis and myeloma share the prevalence of CT7 expression, albeit in a much smaller fraction of cells, whereas other CTAs are mostly absent. Low CTA protein expression may reflect the biological nature of AL amyloidosis resembling pathologic plasma cells with a low proliferative index. Nevertheless, this finding calls into question whether the pathologic plasma cells in patients with AL amyloidosis are truly molecularly identical to plasma cells seen in multiple myeloma.
Treatments for amyloidosis mirror those for multiple myeloma, and, similarly, response rates are high but relapse is frequent. Light-chain amyloidosis differs from myeloma in that patients present with a plasma cell clone that has a low proliferative index and therefore a lower tumor burden.32
The disease is equally devastating, however, because of unique organ dysfunction caused by the amyloidogenic monoclonal proteins released from a small number of abnormal cells. Immunotherapy may hold even more promise than in multiple myeloma, with a small number of malignant cells to target in amyloidosis. While it is reasonable to consider CTAs as a therapeutic option for targeted therapy in amyloidosis, an appropriate next step would be to evaluate the presence of CT7 in order to determine if CT7 expression levels change following treatment.
There was no detectable spontaneous humoral immune response to CT7 or CT10, the most prevalent CTAs in our study. This is not surprising, as CT7 and CT10 have not been particularly immunogenic in prior studies investigating a variety of other tumors. In these previous studies, the most immunogenic antigen was NY-ESO-1; however, NY-ESO-1 expression was not detected in the present series.33, 34
Lack of seroreactivity to CT7 may further reason to induce immune responses against this antigen with immunotherapeutic approaches.
Evidence of seroreactivity to MAGE-A3, NY-ESO-1 or GAGE-2 in individual patients indicates that AL may be considered an immunogenic disease. Unfortunately, no tumor specimen was available for patients with serological responses to MAGE-A3 and NY-ESO-1, and we therefore could not correlate immunogenicity with antigen expression. Of note, the bone marrow specimen from the GAGE-2 seropositive patient was found to be negative for GAGE by IHC. This may be explained by sample error because of known heterogeneity of GAGE expression.
Although protein expression could not be demonstrated in patients with humoral responses to CTAs, it is unlikely that an antibody would be induced without an antigen present. It should be noted that antibody responses to CTA are largely absent in healthy donors. In patients with solid tumors, CTA-specific serum antibody presence is usually correlated with tumor expression of the cognate antigen, and there is a trend for increased immunogenicity of CTA with advanced stages and grades of disease. Finding serum antibodies to CTAs in AL patients is therefore surprising, given the relatively low tumor burden in AL, but it forecasts further evidence of CTA expression in AL amyloidosis.
Taken together, our study demonstrates the prevalence of CT7/MAGE-C1 CTAs are the most commonly expressed in AL amyloidosis. In addition, we demonstrate that there is poor to no expression of other CTAs, such as MAGE-A, NY-ESO-1 or CT10, which are commonly seen in multiple myeloma. On the basis of limited knowledge of CTA biology, we speculate that this could be a reflection of the low proliferation of plasma cells in amyloidosis. Evidence of CTA expression and immunogenicity in AL, albeit not frequent, provides a rationale for further investigation in this disease of potential immunotherapy-based strategies. Because of the low tumor burden, systemic amyloidosis may be susceptible to immunotherapeutic treatment. On the basis of the present findings, CT7/MAGE-C1 appears to be the most reasonable target for future research.