Evidence for adequate thymic function but impaired naive T-cell survival following allogeneic hematopoietic stem cell transplantation in the absence of chronic graft-versus-host disease

Abstract

Allogeneic hematopoietic stem cell transplantation (AHSCT) leads to a prolonged state of immunodeficiency characterized by low peripheral naive T-cell counts. To identify the mechanisms leading to this defect we quantitatively and qualitatively analyzed thymic function through quantification of T-cell receptor excision circle (TREC) frequencies (both the signal-joint TREC [sjTREC] and 6 different D␤J␤ TRECs, by-products of T-cell receptor [TCR] ␣ and ␤ gene rearrangement, respectively), in conjunction with immunophenotype and spectratype analyses in a cohort of patients sampled from 1 to 10 years following AHSCT. In this cohort, reduced thymic function was associated only with ongoing clinical chronic graft-versushost disease (cGVHD). Nonetheless, the diversity of thymic production remained unchanged irrespective of the patient's cGVHD status. Interestingly, increased homeostatic proliferation was found in the naive T-cell compartment of cGVHD ؊ patients who underwent transplantation. However, reduced expression of both the interleukin-7 receptor ␣ (IL-7R␣) (CD127) chain and the antiapoptotic protein Bcl-2 was observed. Taken together, these data indicate that the inability to reconstitute the naive T-cell compartment for several years after AHSCT, in the absence of cGVHD, is a consequence of impaired naive T-cell survival rather than thymic dysfunction. IntroductionAllogeneic hematopoietic stem cell transplantation (AHSCT) is often the only available treatment for several hematologic disorders such as severe aplastic anemia (SAA), acute lymphocytic/ myelogenous leukemia (ALL, AML), chronic myelogenous leukemia (CML), and multiple myeloma (MM). 1,2 The transplantation procedure/conditioning regimen generally leads to a profound and long-lasting state of immunodeficiency characterized by persisting low levels of naive T cells. 3 Pioneer studies aimed at better understanding the mechanisms responsible for the regeneration of the T-cell compartment demonstrated that T cells can be generated through both thymus-dependent and thymus-independent pathways. [4][5][6][7][8][9] However, adequate broadening of the naive T-cell receptor (TCR) repertoire is ensured by the thymus. 10,11 Nowadays, the magnitude of thymic function is best determined through the peripheral blood quantification of T-cell receptor excision circles (TRECs), by-products of TCR gene rearrangement events that occur during thymocyte ontogeny. 12 TREC molecules are extrachromosomal circular DNA generated during V(D)J recombination that do not replicate themselves during mitosis and are diluted-out upon cellular proliferation. As a consequence of this behavior, peripheral blood signal-joint TREC (sjTREC) frequencies are considered surrogate markers for thymic function. Furthermore, peripheral blood quantification of the TREC molecules generated during T-cell receptor beta-chain variable region chain rearrangements (ie, distinct D␤-J␤ or V␤-D␤J␤ TRECs 13 ) illustrates the assortment of the different gene rearrangement events that occurred in the thymus, ...

Immune reconstitution after allogeneic stem cell transplantation with reduced-intensity conditioning regimens

“…Several reports, using phenotypic lymphocyte analysis and/or TRECs quantification, have shown the association between GvHD and delayed T-cell reconstitution after transplantation. [57][58][59][60][61][62] GvHD can affect T-cell recovery in several different ways. First, a number of investigators have shown that GvHD induces thymic dysplasia associated with thymic involution, depletion of cortical and medullary thymocytes, epithelial cell destruction, and loss of Hassall's bodies, which collectively result in T-cell lymphopenia and a failure of negative selection of potentially autoreactive T cells.…”

T-cell reconstitution after allogeneic stem cell transplantation: assessment by measurement of the sjTREC/ TREC ratio and thymic naive T cells

“…6 The number of RTE exported to the periphery is determined by the amount of proliferation which occurs between β-and α-chain rearrangement. 7,8 Normal human T-cell development appears to be broadly similar. 9,10 The information about T-cell neogenesis after stem cell transplantation in humans is much more limited because peripheral blood is the only readily accessible compartment for study and this blood contains only 2% of the body's T cells.…”

Increase of CCR7− CD45RA+ CD8 T cells (TEMRA) in chronic graft-versus-host disease

“…Although it is possible that reduction in size of the T naive and T CM CD8 subsets in cGVHD might be the consequence of the reduced production of naive recent thymic T-cell emigrants and/ Letters to the Editor or impaired survival in the periphery, 6 the mechanisms leading to the increase of CD8 T EMRA cells are unknown.…”

“…5 Recently, a specific activating mutation of the JAK2 tyrosine kinase was detected in 154/201 cases of polycythemia vera (77%), in 21/44 cases of idiopatic myelofibrosis (48%) and in 50/144 cases of essential thrombocythemia (35%). 6,7 The JAK2V617F mutation was subsequently observed only sporadically in 3/119 cases of chronic myelomonocytic leukemia, including a case with a history of B cell lymphoma, and in 5/101 cases of MDS including four patients with a normal karyotype. 8 In our systematic search for genetic abnormalities in t-MDS and t-AML and their association to clinical parameters and cytogenetic characteristics, we examined 140 unselected patients with t-MDS or t-AML previously examined for mutations in the RTK/RAS-BRAF pathway 5 for the JAK2V617F mutation.…”