Phenotypic heterogeneity, novel diagnostic markers, and target expression profiles in normal and neoplastic human mast cells

Mast cells (MC) are specialized immune cells that play a key role in anaphylactic reactions. Growth, differentiation, and function of these cells are regulated by a complex network of cytokines, surface receptors, signaling molecules, the microenvironment, and the genetic background. A number of previous and more recent data suggest that MC are heterogeneous in terms of cytokine-regulation, expression of cytoplasmic and cell surface antigens, and response to ligands. MC heterogeneity is often organ-specific and is considered to be related to MC plasticity, disease-associated factors, and the maturation stage of the cells. The stem cell factor (SCF) receptor KIT (CD117) is expressed on all types of MC independent of maturation and activation-status. In systemic mastocytosis (SM), KIT is often expressed in MC in a mutated and constitutively activated form. In these patients, MC aberrantly display CD2 and CD25, diagnostic markers of neoplastic MC in all SM variants. In advanced SM, MC co-express substantial amounts of CD30, whereas CD2 expression on MC may be decreased compared to indolent SM. Other surface molecules, such as CD63 or CD203c, are overexpressed on neoplastic MC in SM, and are further upregulated upon cross-linking of the IgE receptor. Some of the cell surface antigens expressed on MC or their progenitors may serve as therapeutic targets in the future. These targets include CD25, CD30, CD33, CD44, and CD117/KIT. The current article provides an overview on cell surface antigens and target receptors expressed by MC in physiologic and reactive tissues, and in patients with SM, with special reference to phenotypic heterogeneity and clinical implications.