HMB‐45: a clue to the biology of malignant melanoma?

HMB-45 is a murine monoclonal antibody that was developed by Dr. Allen Gown and colleagues at the University of Washington and which preferentially labels a subgroup ofbenign melanocytic lesions and the majority of malignant melanomas (1). The target antigen for this reagent is still under study, and its physiologic role has been unclear until recently. In keeping with observations that HMB-45 recognizes the junctional component of melanocytic nevi (1) and basal melanocytes adjacent to scars or cutaneous sites of injury (2), it has been postulated that this antibody might select for a population of cells that was actively proliferating. Nonetheless, correlative studies done in our laboratory have compared HMB-45 labelling results with those obtained with anti-PCNA (a proliferation marker analogous to Ki-67) (3) in melanomas, concluding that there were insignificant homologies in the staining patterns of these reagents (4). These data tend to negate the aforementioned hypothesis, and imply that proliferative rate has little to do with binding sites for HMB-45. An alternative postulate has been introduced (2); these authors have amplified their preliminary findings in another paper in this issue (5). Cultured melanocytes were grown under various conditions in vitro, designed to provide information on the effects of trophic factors on HMB-45 immunoreactivity. The authors found that supplemented MCDB-153 medium was extremely effective in promoting strong immunolabelling ofthe cultured cells, whereas basal (unsupplemented) medium was not. Moreover, successive transfers of the cells from supplemented to basal medium, and back again, resulted in sequential diminution and reamplification of HMB-45 binding. In concordance with our own findings, there did not appear to be a strong relationship between tritiated thymidine uptake by the melanocyte cultures and the intensity of immunolabelling, again supporting the contention that proliferation is biologically dissociated from HMB-45 affinity. Smoller et al. suggest that this hybridoma product probably recognizes a target antigen which relates to growth factor responsiveness, and I would certainly concur with such a conclusion. At this point, we may re-examine the nature of supplemented MCDB-153 medium to extrapolate from the relationship between its components and the observations that have just been cited. Which element in the formula of this "cellular soup" is the likely stimulant of HMB-45 antigen synthesis? Insulin, pituitary hormones, and corticosteroids certainly support the growth of melanoma cells in vitro (6), but I am unaware that there is strong clinical evidence for their influence on melanocytic lesions in situ. For example, the literature contains no convincing reports linking hyperinsulinism, hyperpituitarism, or hypercortisolism with the rapid growth of melanomas or melanocytic nevi. The remaining constituent of supplemented MCDB-153 epidermal growth factor is not as easily dismissed in this context. Koprowski and associates have documented the amplification of epidermal growth factor receptor (EGFR) by cultured melanoma cells having two copies of chromosome 7 (7). In addition, this protein is regularly detectable in situ in metastatic melanomas (6), and its density tends to correlate with the morphologic features of such neoplasms (8). Epithelioid, non-pigmented cells preferentially express EGFR, whereas dendritic, melanin-containing elements are typically nonreactive (9, 10). The first of these two descriptions corresponds well to that of Smoller et al., in reference to the cytologic attributes of cells cultured in supplemented MCDB-153. Furthermore, Houghton and colleagues (9, 10) and Real et al. (8) have proposed that epithelioid melanoma cells studied ex vivo are "poorly differentiated" (and presumably actively growing), whereas dendritic components of such lesions are "highly differentiated" and probably have limited growth potential. Unfortunately, it would appear that the molecular integrity of EGFR is highly susceptible to minor variations in specimen handling and fixation (4); consequently, it