Immune pathophysiology of aplastic anemia

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Abstract
Aplastic anemia (AA) remains an elusive disease. Its pathophysiology is not only fascinating by the seemingly simple findings of cytopenia and marrow hypoplasia, but may also contain key information to the understanding of other fundamental processes such as stem cell regeneration, evolution, and immune control of clonal diseases. Although measurements of blood counts provide an objective tool to assess the disease activity and response to the therapy, immune pathophysiology of AA, as inferred from the successes of immunosuppression, provides only few other clinical clues. Similarly, the current laboratory evidence remains mostly indirect. In spite of the recognition of immune pathways of hematopoietic inhibition and apoptosis in AA, the fundamental question about the nature of the antigen(s) inciting or maintaining the pathologic immune response that ultimately leads to bone marrow failure, remains open. However, recognition of the immune targets may aid in understanding not only the pathogenesis but also many of clinical associations and the late squelae of AA. For example, abnormal cells in AA and myelodysplastic syndrome (MDS) MDS may harbor inciting antigens but the immune response lacks selectivity. Clonal selection pressure may be a result of this process or alternatively, emergence of tolerance could lead to the establishment of abnormal hematopoiesis. Clonal proliferation of large granular lymphocytosis could represent an example of an exaggerated response to an immunodominant hematopoietic antigen. In addition to the traditional functional or phenotypic analysis, pathologic immune response in AA can be studied on molecular level by identifying and quantitating T cell clones based on the presence of unique variable B-chain CDR3 sequences. Detection of clonal expansion is based on the observation that in infections and autoimmune conditions, the presence of antigenic drive will lead to the expansion and overrepresentation of T cell clones recognizing this antigen. However, simple analysis of clonal representation is not sufficient to resolve the complex nature of the immune repertoire in the context of genetic and clinical heterogeneity. Therefore, we analyzed VB and CDR3 repertoire in CD4 and CD8 cells, activated or effector cell subsets. To distinguish truly expanded and likely immunodominant clones, we first studied VB distribution and cloned CDR3 sequences from expanded VB families. Identified clonotypic sequences can be used to design molecular tests to quantitate the strength of pathologic immune response. Clonotype sharing has been confirmed in patients with similar clinical features indicating presence of common antigens. In addition, quantitative analysis showed correrlation with the therapy response. Persistence and patterns of clonotypes may be helpful in the classification of immune-mediated marrow failure based on the immune characteristics and will allow inferences into the inciting pathways.