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Anaplastic Large Cell Lymphoma (ALCL) is a peripheral T cell lymphoma of children and adults which is characterized by aberrant ALK expression and signaling, usually driven by the NPM promoter resulting from the t(2;5)(p23;q35). Although ALK+ ALCL cells commonly express T cell activation (CD30) and cytotoxic (Granzyme, perforin, TIA1) markers and have clonal TCR gene rearrangements, the TCR/CD3 signaling complex is strikingly absent. It is also not clear at what stage of T lymphoid development lymphomagenesis initiates, but since NPM expression is ubiquitous it is possible that this may occur before thymic egress.

We undertook a combined human and murine approach to evaluate the place of TCR rearrangement in ALCL development and the timing of NPM-ALK driven lymphoma onset.

Human ALCL were analyzed by multiplex TCR PCR from DNA (d, g and b loci) or cDNA (a locus), with Sanger sequencing and TCRa/d Agilent CGHa analysis for cases with available material. We used the murine model of T cell lymphomagenesis in which NPM-ALK is expressed from the CD4 promoter and following back-crosses to the RAG deficient and/or OTI TCR transgenic lines assessed tumor development and phenotype.

Of 57 human, ALK+ ALCL (48 pediatric, 9 adult) with at least 40% tumor involvement, cases with predominantly in-frame TCRa and TCRb rearrangements were the most common category (56%), followed by 19% with predominantly in-frame TCRg and TCRd rearrangements and 14% with no clonal TCR rearrangements. More surprisingly, 6 cases (11%) demonstrated clonal out-of-frame TCRg but in-frame TCRa rearrangements in the absence of apparent clonal TCRB rearrangement, suggesting that NPM-ALK may in some way replace TCRb mediated signaling.

In keeping with this, CD4 driven NPM-ALK rag competent transgenic mice (CD4NA/rag2^+/+) demonstrated a block in thymic T lymphoid development at the Double Negative (DN)3 stage, which was not observed in the CD4NArag2^-/- mice, which developed cortical thymic tumors composed of CD4/8 DP and CD4 SP cells, but no peripheral tumors. Crossing these mice with the ovalbumin (ova) TCR transgene allowed thymic egress (in the absence of ova) and the development of peripheral ALCL, pathologically similar to those observed in humans. Intriguingly, this only occurred in a RAG competent context and in the absence of ova and was associated with loss of the TCR transgene, since CD4NA/OTImice exposed to ova and CD4NA/rag2^-/- mice only developed non-hematopoietic hepatic or gastro-intestinal sarcomas, although TCR transgene expressing T cells were seen in the peripheryin the former.

These data demonstrate that NPM-ALK (and ALK signaling) allows replacement of the thymic proliferative process of  TCR beta-selection, mediated by the pre-TCR,  and development of thymic tumors, but that TCR expression is required for thymic egress and development of peripheral lymphoma resembling human ALCL. However, the latter is dependenton loss of TCR signaling, suggesting that simultaneous NPM-ALK and TCR activities are not permissive of ALCL development and/or survival. In this context, TCR signaling (but not unstimulated TCR expression) appears to act as a tumor suppressor. These data also represent the first murine model resembling human ALCL and provide proof of concept of an explanation of the pathogenic role for the long recognized “missing TCR” in peripheral T cell lymphomas, which may have implications beyond ALCL.