Increased intensity lymphodepletion and adoptive immunotherapy--how far can we go?

Abstract

In a recent clinical trial involving patients with metastatic melanoma, immunosuppressive conditioning with fludarabine and cyclophosphamide resulted in a 50% response rate in robust long-term persistence of adoptively transferred T cells. Experimental findings indicate that lymphodepletion prior to adoptive transfer of tumor-specific T lymphocytes plays a key role in enhancing treatment efficacy by eliminating regulatory T cells and competing elements of the immune system ('cytokine sinks'). Newly emerging animal data suggest that more profound lymphoablative conditioning with autologous hematopoetic stem-cell rescue might further enhance treatment results. Here we review recent advances in adoptive immunotherapy of solid tumors and discuss the rationale for lymphodepleting conditioning. We also address safety issues associated with translating experimental animal results of total lymphoid ablation into clinical practice.

Figure 1. The rationale for lymphodepletion

(A) In immunoreplete hosts, endogenous cells can inhibit adoptively transferred T cells by at least three mechanisms. (1) T cells, B cells and NK cells compete for homeostatic and activating cytokines such as IL-2, IL-7 and IL-15. (2) T_reg and possibly NK cells and other immune cell populations (e.g. macrophages) display suppressive activity either by direct cell-to-cell contact or release of suppressive cytokines. (3) Immature DCs might fail to activate or might even anergize adoptively transferred T cells, limiting the antitumor response. (4) The tumor itself might act as a negative regulator of T-cell activation by actively producing regulatory molecules and by expressing only limited numbers of major histocompatibility complex–peptide complexes, rendering tumor recognition difficult. (B) Partial (nonmyeloablated) immunodepletion of the host with chemotherapy or total body irradiation before adoptive cell transfer reduces competing ‘cytokine sinks’ as well as regulatory elements. (C) Further increases of immunodepletion to levels that are myeloablative causes profound diminution of inhibitory elements and endogenous ‘cytokine sinks’. The proinflammatory environment activates immature DCs, although their numbers are further reduced. Moreover, conditioning regimen might also alter the tumor itself, promoting the expression of major histocompatibility complex–peptide complexes, increasing the pool of peptides available for presentation and the number of various costimulatory molecules. Thus, alteration of the host environment can lead to an enhanced activation of the transferred T cells, better recognition of the tumor, and ultimately more-efficient tumor destruction. Abbreviations: DC, dendritic cell; IL, interleukin; NK cells, natural killer cells; T_reg, regulatory T cells.

Figure 2. Dose–response functions using a mathematical model described by Sampath et al. based on data collected from clinical trials of bone marrow transplant

Dose–response functions are shown for 1, 2, and 4 fractions combined with 120 mg/kg Cy given over 2 days. A single-dose dose–response function without any Cy is also shown. The arrows highlight the doses of 5.5 Gy and 12 Gy. As shown in the graph, fractionation greatly decreases the incidence of IP at any given dose; in this mathematical model either a 5.5 Gy single dose or a 12 Gy fractionated dose appears to be associated with acceptable IP incidence. Lung shielding is expected to further decrease the risk of IP. Adapted from Elsevier Ltd © Sampath S et al. (2005) Int J Radiat Oncol Biol Phys 63: 876–884. Abbreviations: cy, cyclophosphamide; fx, fraction; IP, interstitial pneumonitis.

Figure 3. Possible applications of adoptive cell transfer using a preconditioning lymphodepleting regimen consisting of chemoradiotherapy and the use of different types of donor T cells in conjunction with administration of exogenous cytokines

(A) The approach used by Dudley et al. in their recent trial. Autologous TILs were isolated from the tumor samples harvested from the patient, tested, expanded and reinfused. (B) The allogeneic cells transferred into the patient could be derived from TILs harvested from another patient (a good responder to autologous adoptive cell transfer) or generated in vitro from peripheral blood lymphocytes of an allogeneic healthy donor and selected for their ability to recognize tumor epitopes in the context of the recipient’s major histocompatibility complex. (C) Effector T cells generated by gene therapy methods using lentiviral or retroviral vectors. TCR sequences can be derived from either allogeneic or xenogenic sources., In the case of xenogenic source, donor animals must express human restriction element (i.e. human leukocyte antigen allele presents in the patient) in order to be able to recognize human antigens on human tissues. Xenogenic donors have a T-cell repertoire not influenced by negative selection against human self-peptide; thus, it may be easier to generate cells with high-avidity TCR for human tumor antigens. In the case of an allogeneic donor a clone expressing a TCR reactive against the patient’s tumor can be generated either in vitro from a healthy donor (i.e. allogeneic allorestricted tumor-specific T cells) or from the TILs of another patient (see B). After isolation of tumor-reactive cells, the TCR is cloned and inserted into a viral vector and used to transduce either autologous or allogeneic PBL, peripheral or cord blood HSC, or even an immortalized T-cell line; the cells are then selected, matured and expanded as needed, and reinfused. In all these scenarios it is also possible to further manipulate cells before transfer by genetic means. This process may include insertion of sequences encoding for cytokines (e.g. interleukins 2 or 15), adhesion molecules, antiapoptotic or suicide genes, and so on. Abbreviations: HSC, hematopoietic stem cells; PBL, peripheral blood lymphocytes; TCR, T-cell receptor; TIL, tumor-infiltrating lymphocyte.