Prospective Molecular Identification of Alloreactive CTL Clones in Allogeneic Hematopoietic Stem Cell Transplantation.

The process of immune recovery after allogeneic HSCT can be characterized by an often profound oligoclonality of the TCR spectrum which may reflect: 1) A decreased diversity within the T cell population or 2) Expansion of individual clones that may be caused by specific antigenic drive exerted by pathogens (e.g., CMV) or alloantigens during the process of GvHD. Novel technologies based on the molecular analysis of the TCR repertoire can be applied to study clonal responses, including multiplex amplification of rearranged TCR VB chains followed by sequencing and quantitation of their contribution to the entire T cell repertoire. We initially studied the T cell repertoire after allogeneic HSCT in sibling (N=20) and matched unrelated (N=9) transplants. VB spectratyping was performed on CD8+ T cells in 22 patients; of the expanded VB families tested, 61.2% (30 of 49) were mono- or oligoclonal by genotyping. The clonal size and structure was determined by sequencing. Immunodominant clones contributed up to 5.4% (avg. 1.4%; range 0.1–5.4%) of all CD8+ T cells, indicating that certain stimuli may drive expansion of immunodominant clones. We originally hypothesized that these expanded clones were allospecific and likely played a role in GvHD; however, we found no correlation between the presence of significant expansions and grade III/IV GvHD. Therefore, in order to identify alloreactive CTL clones and their clonotypic markers, an alternative approach was devised.

The proposed technique utilizes an allostimulation step: recipient cells serve as targets to induce activation of allospecific donor cells. Donor alloreactive cells are identified by their expression of activation markers, such as CD25 or CD69. After sorting, allospecific T cells are used as a source of cDNA for identification and quantitation of allospecific clonotypes. In this fashion, we have analyzed patients undergoing allogeneic sibling and matched unrelated donor grafting (N=7). Prior to transplant, allostimulation was performed and alloreactive CD8-derived clonotypes were subjected to molecular analysis. VB families represented within alloresponsive CTL populations that were oligoclonal by genotyping were subcloned and a large number of CDR3 clones were sequenced to identify the immunodominant clonotypes. Sequences have been derived from activated CD8+ donor cells in 6 cases; an average of 4 (range 1–7) VB families per pair have been characterized.. Although the presence of multiple VB families with a diversified CDR3 spectrum suggests the polyclonal nature of alloresponsive clones, immunodominant clones were identified. A total of 13 immunodominant clonotypes have been identified in 5 patients. Five such clones were identified in one donor/recipient pair; in each pair at least one immunodominant clonotype was isolated. Up to 18 clones per VB family were sequenced, and the average expansion contributed 56% to the entire VB family (range 15–100%).

Clonotype-specific primers have been designed from two expanded clones and used to detect the allospecific clones in post-transplant blood samples in one patient/donor pair. In sum, molecularly defined marker clonotypes indicative of alloresponsive CTLs in HSCT can be individually and prospectively isolated. Such clonotypes may find application in tissue and blood diagnosis of GvHD.