"VSports手机版" Proceedings from the National Cancer Institute’s Second International Workshop on the Biology, Prevention, and Treatment of Relapse After Hematopoietic Stem Cell Transplantation: Part I. Biology of Relapse after Transplantation

Loss of Lymphocyte Populations With Conditioning

There is a generalized loss of lymphocyte populations with most chemotherapy regimens; such loss can be profound after even a single cycle of conventional outpatient therapy. Myeloablative conditioning regimens are highly lymphoablative, leaving only small residual lymphocyte populations. However, following reduced-intensity conditioning, incomplete host lymphoid depletion provides for potential expansion of residual host populations along with donor T cell expansion, adding another layer of complexity to the biology of T cell reconstitution following AlloSCT VSports注册入口.

Limitations to Lymphocyte Recovery After HSCT Conditioning

Some functional lymphocyte and APC populations are regenerated from graft-derived progenitors within a few months, including monocytes, dendritic cells, and NK cells (1, 2). In the absence of B-lymphocyte directed therapy, circulating B cell counts recover within three months, but functional deficits will persist unless T cell subset recovery permits the CD4+ T cell help necessary for memory B cell development. Memory and effector T cells in the graft, particularly CD8+ subsets, can expand in response to antigenic stimulation (3), but regeneration of T cells with a diverse repertoire of T cell receptors (TCR) requires renewed thymopoiesis V体育官网入口. The potential for thymic recovery after HSCT declines sharply with age (4) and may be hindered by tissue damage - from the conditioning regimen as well as, after AlloSCT, acute graft-versus-host disease (GVHD). Resultant thymic epithelial cell (TEC) damage interferes with proper clonal deletion of autoreactive T cells and reduces quantitative T cell production, including the generation of Tregs. Resultant TEC damage may lay the foundation for later development of the autoimmune-like dysfunction of chronic GVHD.

General Pathways for Regeneration of T Cell Subsets (V体育安卓版)

Murine studies identified two pathways of T cell regeneration: thymic-dependent maturation of new T cells from marrow progenitors and thymic-independent expansion of mature peripheral T cells (5–9). Through the latter, T cell numbers are maintained by a balance between their cytokine consumption and levels of constitutively expressed homeostatic cytokines (IL-7 and IL-15) that support their maturation, proliferative expansion and survival. Homeostatic peripheral expansion in lymphopenic hosts can be strongly skewed by rapid, antigen-driven proliferation VSports在线直播. Translational studies in children, which validated CD45 isoform-defined CD4+ T cell subsets, demonstrated recovery of total and naïve (CD45+CD45RO−) peripheral CD4+ T cells as early as six months following severe chemotherapy-induced lymphopenia, and established thymic-dependent T cell production as primarily responsible for this repopulation in young patients (6). In contrast, in adults, few naïve peripheral CD4+ T cells are generated during the first year after chemotherapy, while circulating CD4+ T cells with a memory phenotype (CD45RO+) recover to nearly pretreatment levels (~83%) within three months through thymus-independent peripheral expansion (10).

Longitudinal study of T cell recovery in adults after myeloablative conditioning and AHSCT for breast cancer demonstrated the influence of age-associated thymic renewal on CD4+ T cell recovery. In patients in whom renewal of thymopoiesis was observed – as assessed by naïve cell phenotype (CD45RA+), T cell receptor excision circle (TREC) frequency, thymic size and rediversification of TCR repertoire – normalization of circulating CD4+ T cell counts was noted by the end of the second year and permitted reestablishment of central memory populations. In those who did not reactivate thymopoiesis, CD4+ T cell counts remained below normal even up to five years after AHSCT (4). Recovery of CD8+ T cells in the same patient cohort followed markedly different repopulation kinetics, with three distinct patterns observed (Figure 1). In more than half of the patients, CD28−CD8+ T cell expansion yielded a massive early spike in total peripheral CD8+ T cell counts, and constituted the dominant population in persistently elevated CD8+ T cells for two years after AHSCT. The circulating CD8+ T cell population demonstrated limited TCR repertoire diversity, with a high proportion of oligoclonal, expanded, CD28− cells by TCR spectratyping that was strongly associated with a history of CMV infection, suggesting viral antigen-driven peripheral expansion. In younger adult patients, CD8+ T cell recovery bore resemblance to that of CD4+ T cells: phenotypically naïve CD8+ T cells gradually predominated the circulating CD8+ T cell pool during the second year, generating a population with a broadly diverse TCR repertoire and high TREC frequencies resulting from thymic reactivation. A subset of younger patients also demonstrated the early, oligoclonal expansion of CD8+ T cells, suggesting that peripheral and thymic pathways were independent modulators of recovery. A third pattern of CD8+ T cell repopulation was characterized by the absence of both the early expansion of effectors and the later recovery of naïve CD8+ T cells; these patients experienced prolonged, severe deficits in circulating T cell numbers and in reconstitution of TCR repertoire diversity in CD4+ as well as CD8+ T-cell subsets (Fig. 1). This third pattern exhibits the limits of thymus-independent homeostatic expansion in reconstituting functional immune competence in middle-aged adults V体育2025版.

The regeneration of Tregs is less well characterized than for the conventional T cell subsets described. It appears that Treg repopulation also can derive from thymus-dependent and peripheral expansion pathways in the setting of lymphopenia (11), and that thymic renewal may yield Treg populations capable of modulating chronic GVHD (12). Exogenous IL-2 administration increases the number of circulating Tregs through both pathways (13), with therapeutic potential for prevention and treatment of chronic GVHD (14).

Implications for the Prevention and Treatment of Relapse

A better understanding of physiologic regulation of thymic function is needed to identify clinical strategies to optimize the thymus-dependent pathway, particularly in adults. Insulin-like growth factor-1, androgen blockade and keratinocyte growth factor are candidate regulators amenable to clinical approaches to enhance recovery of thymopoiesis and of functional T cell immunity. Further, clinical availability of primary homeostatic cytokines (i.e., IL-7 and IL-15) (21) creates the possibility of enhancing the peripheral expansion pathway and tumor-antigen driven expansion after HSCT and/or in conjunction with tumor-specific immunotherapies. Similarly, our developing understanding of the biology of Treg homeostasis may yield opportunity for relapse prevention through GVHD modulation (e.g., IL-2). As functionally distinct T cell subsets continue to be identified, some, e.g., “memory stem T cells” (15), may be important determinants of functional immune reconstitution following recovery from lymphopenia. The biology of emerging T cell subsets will need to be integrated into our understanding of T cell homeostasis and immune reconstitution.

Recent advances in cellular engineering create potentially powerful tools for generating cancer-specific immune responses. The development of genetically modified effector T cells now permits deliberate skewing of the T cell repertoire toward antitumor effector populations. Chimeric antigen receptors redirect T cell specificity and function through combining a specific antibody’s antigen-recognition domain with a T cell signaling domain yielding a receptor with high specificity and independence from TCR-MHC restriction (16). The biology of homeostatic antigen-driven peripheral expansion may be exploited to favor proliferation of adoptively transferred cells; timing administration to coincide with therapy-induced lymphopenia and/or coadministration of exogenous homeostatic cytokines are potentially synergistic immunomodulatory strategies to support their survival and expansion in vivo. Interruption of counter-regulatory mechanisms may improve antitumor responses as well, with Treg depletion and checkpoint inhibition (e.g., PD-1 (17)) being examples of promising strategies. AHSCT-associated lymphopenia also provides an advantageous milieu for homeostatic expansion and an opportunity to support antigen-directed immunotherapy by vaccines or other immunomodulatory approaches. Remarkable recent progress in employing such strategies was discussed in Sessions II and III (49–50). Perhaps for the first time in the history of hematopoietic stem cell transplantation, we have arrived at an intersection of knowledge and technology that may permit removal of the fundamental barriers that have limited this field from its inception.

"V体育2025版" Functional Activity of NK Cells Reconstituting Early After Transplantation

Following AlloSCT, NK cells are the first lymphocyte population to recover, with donor cells predominant. NK cells are attractive to exploit in the setting of HSCT because their early recovery occurs when the adaptive immune system is particularly weak. However, the repopulating NK cells that are circulating early after AlloSCT are phenotypically and functionally different from the highly functional, FcRγIII-bearing CD56^dim NK cells found in peripheral blood from healthy donors. These early NK cells may be developmentally immature, with diminished KIR expression and higher expression of NKG2A (25) and CD56. CD56^bright NK cells are a normally small population in the periphery (usually found in secondary lymphoid tissues and the pregnant uterus); while they respond to IL-12 and IL-18 with high IFNγ production, they lack cytotoxic effector capabilities. NK cell responses to targets are defective, especially for target-induced cytokine production (26). These responses normalize over the first year following HSCT, coinciding with T cell recovery and suggesting that T cells may play a role in the acquisition of NK cell maturation and cytotoxic function, possibly through the cytokines they release.

Endogenous Pathways and Pitfalls to Enhanced NK Cell Function

Based on murine studies with CMV and Ly49H⁺ NK cells, human studies suggest that the C-type lectin-like receptor NKG2C and the KIR family may be involved in viral infection. We have studied NK cell function after human CMV reactivation following umbilical cord blood (UCB) AlloSCT (27). In this setting, recipient CMV reactivation would recapitulate a primary immune response from the CMV-naïve donor NK cells. Patient blood samples were collected at the time of reactivation and at two, four and eight weeks after initiating antiviral therapy. NKG2C⁺ NK cells increased following detection of virus in the blood, peaking four weeks after reactivation. NK cells expressing NKG2C produced significantly more IFNγ after exposure to a Class I⁻ target than NK cells lacking NKG2C. Four weeks after reactivation, NK cells coexpressing NKG2C and KIR produced significantly more IFNγ than NK cells expressing KIR but not NKG2C. In the context of education of KIR⁺ NK cells through their MHC ligands, only NKG2C⁺ NK cells with self-KIR (i.e., NK cells that express KIR where the cognate ligand is expressed in the recipient) made IFNγ; in contrast, NKG2C⁺ nonself KIR⁺ NK cells were hyporesponsive. Remarkably, an expanded mature, highly functional NKG2C⁺KIR⁺ NK cell population persisted throughout the first year after transplant. These findings support the emerging concepts of NK cell memory in humans and that NK cell responses to CMV reactivation may promote more generalized recovery of NK cell maturation and functional competence that could be important for protecting against cancer relapse and/or viral infections.

Trials exploring adoptive transfer of autologous activated NK cells have not demonstrated efficacy, presumably because of inhibition through Class I receptor signaling (28). Reports of the benefit of allogeneic NK cells after AlloSCT with donors expressing nonrecipient (i.e., nonself) inhibitory KIR led to exploration of allogeneic NK cell adoptive transfer. In order to overcome barriers of allogeneic immunity and achieve in-vivo NK cell expansion, success was dependent on a potent lymphodepleting regimen to 1) create space; 2) increase endogenous cytokines; and 3) to eliminate immune rejection of adoptively transferred cells. In 2005, we identified the requirement for lymphodepleting chemotherapy to promote in-vivo expansion of NK cells after adoptive transfer of haploidentical related-donor NK cells in a nontransplant setting (29). NK cells were infused with IL-2 administration for the two-week interval after adoptive transfer. Chimerism studies established unequivocally that donor-derived haploidentical NK cells could persist and expand in vivo in some patients; antitumor responses seemed to correlate with in-vivo NK cell expansion. In some settings, IL-2 administration may result in Treg expansion and contraction of the immune response (30, 31). In an attempt to further overcome these barriers, some investigations have been exploring increased lymphodepletion by adding total body irradiation, approaching myeloablative intensity conditioning. In this setting, HSCT may be warranted to avoid prolonged neutropenia. Clearly, safer strategies are needed to maximize the effector-to-target ratio in vivo. IL-15 administration has recently shown promise in primates; it is being produced by the NCI and clinical trials are in progress. The main advantage of IL-15 is that it does not stimulate CD25^hi (IL-2Rα) Treg expansion. An alternative approach is ex-vivo expansion of NK cells, which could permit infusion of a greater cell dose and, potentially, reduced dependence upon potent, toxic conditioning for lymphodepletion.

Future Directions in Optimizing NK Cell Therapeutics

Our understanding of NK cell-target interactions requires more detailed exploration of the potency of: specific NK molecules and their ligands on specific target cells (including malignant stem cells); the stages of NK maturation; and the effect of the recipient milieu on NK cell maturation, survival and clonal selection. Strategies to overcome tumor-induced immunosuppression and allogeneic rejection of adoptively transferred cells will likely be needed. Targeting NK cells to increase their specificity may also enhance efficacy. We have recently been exploring the potent signal delivered through CD16. Bi- or tri-specific killer-cell engagers (“BiKEs” and “TriKEs”) - comprised of one or two single-chain Fv for a target-specific antigen and a single-chain Fv capable of triggering CD16 - seem to be especially potent for this purpose (32). Further studies will determine the most favorable context for NK cell therapy, how best to promote in-vivo survival needed for clinical response and how to optimize their specificity in a minimal-residual-disease setting after transplantation.