Abstract
Acquired Severe Aplastic Anemia (aSAA) is a rare disease characterized by pancytopenia and bone marrow hypocellularity. There is good clinical and laboratory evidence that a T-cell mediated autoimmune mechanism directed against stem- and progenitor cells located in bone marrow plays a major role in the pathogenesis of aSAA. Immunosuppressive therapy (IST) and bone marrow transplantation (BMT) are the two treatment options. One of the key issues selecting the appropriate treatment is the long latency period of 4–6 months before a response to IST. In a large multicenter prospective study in children we could show that the severity of the disease (i.e. degree of granulocytopenia < 200/μl) is predictive for response to IST. The spectratyping (immunoscope)-technique reflects the T-cell diversity by determining the T-cell receptor (TCR) CDR3 length polymorphisms. Since the grade of polymorphism is proportional to the number of peaks detected, it is possible to determine a complexity score which reflects the diversity of the T-cell population. Herein, we prospectively analyzed the clonality of Vbeta TCR-repertoires in bone marrow of children with aSAA (n=6). After treatment with IST we correlated the initial diversity of the T-cell repertoire with their response 4–6 months after the start of IST. Moreover, we addressed the question weather the bone marrow T-cell repertoire is skewed by the expansion of specific cells, which indicates an antigen driven immune response located in BM. For that we compared the complexities of bone marrow (BM) and respective peripheral blood (PB) derived lymphocytes. The Vbeta profiles in bone marrow compared to PB were skewed in aSAA patients (n=4) most apparent in the CD8 population (figure). The complexities in the healthy control group (n=3) were similar in both compartments. Furthermore, in patients with a haematological response (n=3) a decrease in complexity of T-lymphocytes was present in BM compared to IST non-responders (n=3). The decreased complexity reflects a high number of monoclonal expansions in BM as determined by sequencing analysis of 37 (CD4) and 55 (CD8) CDR3 regions. However, we could not detect preferential usage of specific Vbeta families, J-beta segments or uncommon features in CDR3 size compared to V(D)J-rearranged sequences in the NCBI Prot database. Our findings presented here provide evidence for BM specific proliferation of lymphocytes possibly due to an antigen driven immune response. Moreover, the TCR complexity at diagnosis is associated with a later IST response and could thus be used as an additional predictor for the appropriate treatment.
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