T Cell Exhaustion/Senescence in Relapsed Multiple Myeloma after Autologous Stem Cell Transplantation

BACKGROUND: Multiple myeloma (MM) is the most common indication for high-dose chemotherapy and autologous stem cell transplantation (ASCT).  Post-transplant lenalidomide maintenance therapy doubles progression-free survival, but almost all patients eventually relapse.  The immune system participates in the control of MM, whereas compromised immunity contributes to its evolution.  Post-transplant immunotherapy to induce or restore antitumor immunity offers a promising approach to target residual MM and improve patient outcomes.  The rational development of immunotherapeutic interventions after ASCT, however, requires a comprehensive understanding of the immunologic milieu.  We therefore evaluated lymphocyte composition and function after ASCT to guide optimal timing of immunotherapy and to identify potential markers of relapse. 

METHODS: Fifty-five MM patients undergoing ASCT were evaluated for at least one year.  Peripheral blood from patients was obtained before ASCT and on d +12, +30, +90, +180, and +365 after ASCT, and at the time of relapse where applicable.  Leukocyte concentrates were used as a source of healthy donor cells.  Mononuclear cells were analyzed by flow cytometry for phenotypic assessment of lymphocyte subset composition.  Functional assessment of dendritic cell and T cell activity in vitro was assayed in autologous and allogeneic mixed leukocyte reactions, cytotoxic T lymphocyte (CTL) lysis assays, and PD-1 blockade experiments.

RESULTS: CD3⁺CD4⁺CD25^bright CD127^neg regulatory T cells (Tregs) decline as CD8⁺ T cells expand during early lymphocyte recovery after ASCT, markedly reducing the Treg:CD8⁺ effector T-cell ratio (Fig 1A) and providing a critical early window for the introduction of immune-based post-transplant consolidation therapies.  CD8⁺ T cells can respond to autologous dendritic cells presenting tumor antigen in vitro as early as day +12 post-transplant, becoming antigen-specific CTL effectors and thereby demonstrating preservation of cellular reactivity (Fig 1B).  CD4⁺ and CD8⁺ T cells express the negative regulatory molecules, CTLA-4, PD-1, LAG-3, and TIM-3, before and after ASCT (data not shown).  A subpopulation of exhausted/senescent CD8⁺ T cells, however, down-regulates CD28 (Fig 2A) and up-regulates CD57 (Fig 2B) and PD-1 (Fig 2C), characterizing immune impairment and relapse after ASCT.  CD4⁺ T cells show the same trends in the frequencies of CD28^neg, CD28^negCD57⁺, and CD28^negPD-1⁺ cells, albeit at lower levels of expression (Fig 2D).  Relapsing patients have higher numbers of CD8⁺CD28^negPD-1⁺ T cells at +3 months after transplant (Fig 2E), but before detection of clinical disease, indicating their applicability in identifying patients at higher risk of relapse.  PD-1 blockade revives the proliferation and cytokine secretion of the hyporesponsive, CD8⁺CD28^negPD-1⁺ T cells in vitro (Fig 3). 

CONCLUSION: These results identify T cell exhaustion/senescence as a distinguishing feature of relapse and support early introduction of immunotherapy to stimulate antitumor immunity after ASCT.