Non‐self‐discrimination as a driving concept in the identification of an immunodominant HMW‐MAA epitopic peptide sequence by autoantibodies from melanoma cancer patients

Abstract

We analyzed the sera of patients with melanoma to define the human humoral autoantibody profile towards HMW-MAA. Computational proteome scanning using the non-selfdiscrimination principle as a guide led to the individuation of the low-similarity HMW-MAA 781 The vast majority of melanomas develop from cutaneous intraepidermal melanocytes either in normal skin or within melanocytic nevi and progress through radial and vertical growth phases. Cutaneous melanoma accounts for about 10% of skin cancer cases but 75% of skin cancer deaths. The incidence of cutaneous melanoma is rising steadily worldwide, and the median survival of patients with distant metastases remains Ͻ1 year. 1,2 Adolescent cases are on the rise too: the incidence of melanoma increased by 85% among 15-to 19-year-olds from 1973 to 1996. 3 This situation demands efforts to develop new biologic therapies. 4 A promising approach is the use of specific immunotherapy with peptide-based vaccine strategies. Conditio sine qua non to this goal is the identification of immunogenic tumor antigens, including antigenic peptides able to evoke humoral/cellular responses. [5][6][7] Melanoma presents a heterogeneous pattern of various tumor markers, including melanocytic antigens and cancer testis antigens. 8 Among them, HMW-MAA 9 is a melanoma marker of particular interest since (i) it is highly expressed at the surface of tumor cells, (ii) it has restricted distribution in normal tissues 10 -12 and (iii) the induction of specific humoral response to anti-idiotypic anti-HMW-MAA MAb increases survival in patients with advanced melanoma. 13,14 Given these premises, identification of the epitopic sequences of this proteoglycan MAA is of great interest for obtaining specific antibodies to activate the destruction of melanoma cells. However, the dimensions of the HMW-MAA molecule do not allow the definition of HMW-MAA conformational determinants and hamper the exact individuation of the immunogenic linear peptide portions of this antigen through the usual epitope mapping methodology.We have used computational biology to exploring TAA epitopes recognized by MAbs or polyclonal antibodies. [15][16][17][18] By measuring the antigenic peptide similarity to the host's proteome, we demonstrated that a low level of similarity is a necessary condition for a peptide to be immunogenic. Consequently, based on the assumption that antigenic motifs that are scarcely represented in human proteins may offer epitopic determinants unknown to the host, we searched for immunogenic epitopes of the human melanomaassociated chondroitin sulfate proteoglycan by, first, selecting for portions of the HMW-MAA not shared with the human proteome and, then, analyzing the low-similarity peptide reactivity pattern using sera from 26 melanoma patients.

The self/nonself issue: A confrontation between proteomes

“…The data add to and support experimental results from our laboratory [21][22][23][24][25][26][27]31 and reported meta-analyses. [28][29][30] Theoretically, proteomic similarity analyses might elucidate the regulatory mechanisms/factors that dictate peptide immunogenicity assessment.…”

“…Using the available proteome repertoire, we advanced and explored the hypothesis that the immunogenicity of peptide sequences is modulated by low similarity to the host's proteome. During the past decade, a series of experimental models involving different disease-associated proteins [20][21][22][23][24][25][26][27][28][29][30][31] have substantiated and supported the low-similarity hypothesis.…”

“…Then, each pentapeptide was analyzed for the number of occurrences in the host proteome using described methodologies. [20][21][22][23][24][25][26][27][28][29][30][31] Any occurrence is called a match. A pentapeptide with up to five perfect matches to the host proteome was considered a low-similarity sequence.…”

Proteomic Scan for Tyrosinase Peptide Antigenic Pattern in Vitiligo and Melanoma: Role of Sequence Similarity and HLA-DR1 Affinity

“…Scanning of the amino acid sequence of the melanocyte-associated-protein tyrosinase for regions with no or low similarity to the human proteome was conducted by using the following computational procedure. Because pentapeptide units can be antigenic sites sufficient to host minimal antigenic determinants (24 -26), the human tyrosinase sequence was dissected into pentamers that were used against human proteome in similarity analyses by using exact peptide match program (15)(16)(17)(18)(19). Pentamers overlapped by four residues, i.e., MLLAV, LLAVL, LAVLY, AVLYC, VLYCL, etc., were sequentially used.…”

“…The underlying scientific rationale is that immune system is allowed to respond only to rarely encountered/never seen antigenic sequences (15)(16)(17)(18)(19). To test our hypothesis in this study, synthetic peptides corresponding to low similarity sequences present in the tyrosinase autoantigen were immunoassayed by using vitiligo and melanoma sera.…”