Children with severe viral infection due to primary or secondary immunodeficiency and children with cancer are the primary pediatric populations for which T cell based immunotherapies are being developed. HSCT is a common cause of secondary immunodeficiency since it induces severe lymphocyte depletion, which typically lasts at least one year and may persist for several years following the procedure. Furthermore, common therapies for childhood cancer induce profound lymphocyte depletion and significant immunosuppressive effects result from cancer itself. Thus, patients receiving T cell based immunotherapies have alterations in host immunity that can impact the effectiveness of T cell based immunotherapy both positively and negatively. This section will describe the changes in immune physiology induced by T cell depletion and discuss the effects that these and other host factors play in enhancing or diminishing the effectiveness of T cell based immunotherapies for cancer or viral infection.
Unlike other marrow-derived populations, B cells and T cells require specialized microenvironments within the bone marrow and thymus respectively, to recapitulate primary development. The bone marrow microenvironment needed to support B cell lymphopoiesis remains functional throughout life, however, age-related changes occur within the thymus that limit the capacity for postnatal humans to regenerate T cells62
. Many investigators have emphasized the importance of puberty and sex steroids in age associate thymic involution, but in fact, from birth onward there is a relatively linear decline in the relative mass and function of the thymus. As a result, adolescents have substantially diminished thymic function compared to younger children63
, and the majority of patients in the fifth decade of life essentially show a complete inability to recover T cells via thymic-dependent pathways after T cell depletion64
. Furthermore, even in very young children, the thymic microenvironment is exquisitely susceptible to damage by a variety of insults, including cytotoxic agents, viral infections, GVHD, and irradiation65
, thus limiting that capacity for even young children with cancer or immunodeficiency to support thymic-dependent T cell regeneration.
When thymic-dependent T cell regeneration is limiting, T lymphocytes can be partially regenerated by thymic-independent homeostatic peripheral expansion
. This process substantially increases T cell numbers and immune function, but it does not fully restore immune competence. Briefly, mature T cells, (either remaining within the host following the lymphopenia inducing insult, emerging from a diminished thymus, derived from maternal T cells or adoptively transferred through a stem cell graft or immunotherapy product) undergo vigorous mitotic expansion, which is dramatically enhanced compared to low level cycling that T cells normally undergo throughout life in the absence of lymphopenia. This cycling represents a combination of enhanced T cell proliferation toward cognate antigens (e.g. viral antigens present during lymphopenia)66
, T cell proliferation in response to cross reactive antigens expressed by commensal flora in the gut, and T cell proliferation toward self-antigens, which do not induce substantial T cell cycling under lymphoreplete conditions, but can induce marked T cell proliferation in the setting of lymphopenia67
. Thus, lymphopenia results in profound increases in global T cell cycling and increased responsiveness to antigens. These alterations in immune reactivity are primarily driven by interleukin-7 (IL-7), a stromal cell derived product that is a primary regulator of T cell homeostasis.
IL-7 is produced by non-lymphocytes including stroma within lymphoid tissues, and parenchymal cells in the skin, gut, kidney, etc. Nearly all T cells express the IL7 receptor and continually utilize this cytokine for survival68
. When T cells are depleted, less IL-7 is utilized and IL-7 levels increase through accumulatation69
. Normally, young children maintain serum IL-7 levels of 10–20 pg/ml, whereas healthy adults maintain IL-7 levels of 2–8 pg/ml. However, during lymphopenia, IL-7 levels increase to as high as 60 pg/ml. Rises in serum IL-7 levels in clinical settings associated with lymphopenia have been described following bone marrow transplantation, in human immunodeficiency virus (HIV) infection, following chemotherapy for cancer and in idiopathic CD4 lymphopenia. The increased availability of IL-7 drives the dramatic T cells cycling that occurs during lymphopenia (termed homeostatic peripheral expansion or HPE). Furthermore, treatment of non-lymphopenic mice, monkeys and humans70
with recombinant human IL-7 (rhIL-7) induces increases in T cell cycling (and, subsequently, T cell number) that closely resemble that seen during lymphopenia.
HPE efficiently increases T cell numbers, but does not generate new T cell specificities from HSCs, and therefore the T cell receptor repertoire of populations generated via this pathway remains limited, especially when depletion is severe. Furthermore, patients reliant on HPE for T cell regeneration have chronically diminished CD4+ counts, diminished CD4/CD8 ratios, and diminished numbers (but higher proportions) of suppressive CD4+ T cells. Therefore, the changes in immune physiology induced by T cell depletion enhance T cell reactivity but also results in chronic immune deficiencies. From an immunotherapist’s perspective, these changes are potentially exploitable, especially in the context of adoptive immunotherapy, which requires efficient expansion of adoptively transferred T cells. Indeed, recent non-randomized studies have suggested that induced lymphocyte depletion may actually enhance the efficacy of adoptive immunotherapy for cancer. Dudley et al, administered autologous tumor infiltrating lymphocytes harvested from patients with melanoma, expanded ex vivo and reinfused with rhIL-2 to patients with or without regimens to induce lymphopenia. In sequential non-randomized trials, they observed progressive increases in tumor response rates associated with increasing degrees of lymphocyte depletion. Similar results were seen in animal studies and in clinical trials wherein monoclonal antibodies targeting CD45 to induce lymphopenia appeared to augment the effectiveness of adoptive immunotherapy for nasopharyngeal carcinoma52
. Thus, children who experience lymphocyte depletion due to congenital or acquired immunodeficiency, HSCT, or as a result of dose intensive chemotherapy for cancer, may be good candidates for T cell based therapies because the lymphopenia associated with their underlying disease can serve to increase the effectiveness of adoptive cell therapy.
Importantly, however, there are significant short and long-term toxicities associated with lymphopenia. Moreover, when the immunotherapy administered incorporates vaccines, which rely of endogenous T cells present within the host to mediate immune responses, chronic lymphopenia and limited repertoire diversity induced by T cell depletion may actually diminish the effectiveness of immune based therapies. This impact of reduced T cells number and restircted repertoire has been demonstrated in animal studies wherein lymphopenia diminishes the ability to control micrometastatic disease in cancer. Thus, future work seeks to replicate the beneficial aspects of lymphopenia in supporting T cell based immunoptherapy while avoiding the detrimental effects. This approach has been effective in animal studies, where targeted therapies that specifically deplete suppressive T cells and utilize rhIL7 to replicate the lymphopenic milieu in lymphoreplete hosts resulted in better outcomes following adoptive immunotherapy than when the same therapy was administered to lymphopenic hosts71
Putting it all together, there is great interest in incorporating immune based therapies into existing standard therapies for childhood cancer. Since it is not uncommon for children with high-risk tumors to be rendered free of visible disease using standard multimodality therapy and since such patient populations are also profoundly lymphopenic upon completion of dose intensive therapy, this provides a certain “window of opportunity” for treating minimal residual disease in patients with high risk cancers. Indeed, “consolidative immunotherapy”, which combines tumor vaccines with therapies to enhance immune reconstitution has been piloted in patients with high-risk pediatric sarcomas. 72
Briefly, patients with metastatic and recurrent Ewing sarcoma and alveolar rhabdomyosarcoma undergo apheresis for collection of T cells prior to initiation of therapy. Following treatment with standard dose intensive chemotherapy and local therapy to attempt to induce a state of minimal residual disease, they receive infusion of autologous T cells as a source for homeostatic peripheral expansion and sequential tumor vaccines using dendritic cells. This approach demonstrated favorable survival using an intent-to-treat analysis, however conclusions regarding efficacy are hampered by issues of selection bias, and the lack of a randomized control arm. Despite these caveats, the study clearly demonstrated that all immunized patients, regardless of profound lymphopenia present at the time of vaccination, demonstrated the capacity to generate T cell responses to vaccination within 3 months following chemotherapy, indicating that vaccine induced T cell responses can be induced early after cytotoxic chemotherapy when combined with autologous T cell infusions. A subsequent study targeting patients with metastatic and recurrent pediatric sarcomas is underway with a modified DC vaccine, which incorporates approaches to deplete regulatory or suppressive CD4+ T cells and also incorporates rhIL-7 to enhance immune reconstitution.
In summary, an increased understanding of the biology of T cell mediated antiviral responses and tumor/ immune interactions have opened up real opportunities to harness T cells for clinical benefit in children with immunodeficiency associated infections and in children with cancer. Conceptually, the critical elements have been defined and clear proof-of-principle has been demonstrated. However, substantial work is needed to optimize these therapies, to broaden their applications beyond infection and enhance the effectiveness of tumor directed therapies and to simplify their administration so that they can be tested in large, controlled randomized studies. It is clear that if T cells are to be effective therapy for malignancies, CTLs must proliferate in vivo following infusion, whilst retaining their anti-tumor activity. Optimal proliferation depends on infusing T cells to an environment that promotes homeostatic expansion. The lymphopenia associated with post HSCT environment is similar to that in which autologous immunotherapy has been utilized. In addition with the emerging methodologies available to detect relapse following HSCT, there will be increasing numbers of patients who may benefit from these immunotherapeutic approaches instead of or as an adjunct to the non-specific graft versus tumor effect discussed elsewhere in this edition. Furthermore, infectious complications of HSCT are more frequent following T cell depleted allografts (also discussed elsewhere in this edition) for which infectious pathogen-specific adoptive therapies will play an important role. With increased knowledge of the optimum methodology for generation of T-cell products, and optimization of approaches to enhance the function of adoptively transferred, adoptive immunotherapy strategies may find increasing use to reduce the risk of relapse and prevent and treat infections post transplant.