Prevention of murine graft-versus-host disease by inducing and eliminating ASGM1+ cells of donor origin

Immune reconstitution after T-cell replete HLA haploidentical hematopoietic stem cell transplantation using high-dose post-transplant cyclophosphamide

As HLA haploidentical related donors are quickly available, HLA haploidentical hematopoietic stem cell transplantation (haploHSCT) using high-dose post-transplant cyclophosphamide (PTCy) is now widely used. Recent basic and clinical studies revealed the details of immune reconstitution after T-cell replete haploHSCT using PTCy. T cells and NK cells in the graft proliferate abundantly at day 3 post-haploHSCT, and the PTCy eliminates these proliferating cells. After ablation of proliferating mature cells, donor-derived NK cell reconstitution occurs after the second week; however, recovering NK cells remain functionally impaired for at least several months after haploHSCT. PTCy depletes proliferating cells, resulting in the preferential accumulation of Treg and CD4+ T cells, especially the memory stem T cell (T_^SCM) phenotype. T_^SCM capable of both self-renewal and differentiation into effector T cells may play an important role in the first month of immune reconstitution. Subsequently, de novo T cells progressively recover but their levels remain well below those of donor CD4+ T cells at the first year after haploHSCT. The phenotype of recovering T cells after HSCT is predominantly effector memory, whereas B cells are predominantly phenotypically naive throughout the first year after haploHSCT. B cell recovery depends on de novo generation and they are not detected until week 4 after haploHSCT. At week 5, recovering B cells mostly exhibit an unconventional transitional cell phenotype and the cell subset undergoes maturation. Recent advances in immune reconstitution have improved our understanding of the relationship between haploHSCT with PTCy and the clinical outcome.

Natural Killer Cells in Graft-versus-Host-Disease after Allogeneic Hematopoietic Cell Transplantation

Allogeneic hematopoietic cell transplantation (HCT) is a well-established therapeutic modality effective for a variety of hematological malignancies but, unfortunately, is associated with significant morbidity and mortality related to cancer relapse as well as to transplant-related complications including graft-versus-host-disease (GvHD). Natural killer (NK) cells are the first donor-derived lymphocyte subset to recover after HCT, and their crucial role in protection against cancer relapse and infections is well established. Conversely, the role played by NK cells in GvHD is still controversial. Early studies suggested a participation of NK cells in GvHD induction or exacerbation. Subsequently, experimental evidence obtained in mice as well observational studies performed in humans led to a model in which NK cells play a regulatory role in GvHD by repressing alloreactive T cell responses. This widely accepted model has been recently challenged by clinical evidence indicating that NK cells can in some cases promote GvHD. In this review, we summarize available knowledge about the role of NK cells in GVHD pathogenesis. We review studies uncovering cellular mechanisms through which NK cells interact with other immune cell subsets during GvHD leading to a model in which NK cells naturally suppress GvHD through their cytotoxic ability to inhibit T cell activation unless exogenous hyperactivation lead them to produce proinflammatory cytokines that can conversely sustain T cell-mediated GvHD induction.

Evaluation of interleukin 12 and CD56+ lymphocyte cells in pediatric hematopoietic stem cell transplantation for early diagnosis of acute graft versus host disease

The present study tried to explain CD56+ lymphocyte cells activities and possible prognostic role of these cells in Graft-Versus-Host-Disease (GVHD). The role of IL-12 activation and function is of interest in this study. Peripheral blood samples of 51 Hematopoietic Stem Cell Transplantation (HSCT) recipients collected at before (day -8) and after (days 7 and 14). PBMC were collected by Ficoll separation and analyzed by Flow Cytometry using triple antibody (CD45-PerCP, CD56-FITC, and CD69-PE staining and control antibody. Levels of the cytokine IL-12 in the patient's serum were evaluated by ELISA. Percentage of CD56+ lymphocytes (CD56+^bright) cells was significantly increased at day 14 in patients with acute GVHD and percentage of lymphocytes expressing CD69 was significantly increased at days 7 and 14 posts HSCT in patients with acute GVHD in comparison to those in non-GVHD patients. Baseline serum IL-12 levels (pre-HSCT, day -8) were significantly higher in those HSCT recipients who did not develop GVHD. This study showed that post-transplant CD56+ lymphocytes and pre-transplant serum levels of IL-12 play significant roles in the induction of and protection against GVHD, respectively. The increase in the percentage of CD69+ cells indicates the activation of lymphocyte in acute GVHD group.

Acute GVHD in patients receiving IL-15/4-1BBL activated NK cells following T-cell-depleted stem cell transplantation

Natural killer (NK) cells can enhance engraftment and mediate graft-versus-leukemia following allogeneic hematopoietic stem cell transplantation (HSCT), but the potency of graft-versus-leukemia mediated by naturally reconstituting NK cells following HSCT is limited. Preclinical studies demonstrate that activation of NK cells using interleukin-15 (IL-15) plus 4-1BBL upregulates activating receptor expression and augments killing capacity. In an effort to amplify the beneficial effects of NK cells post-HSCT, we conducted a first-in-human trial of adoptive transfer of donor-derived IL-15/4-1BBL-activated NK cells (aNK-DLI) following HLA-matched, T-cell-depleted (1-2 × 10(4) T cells/kg) nonmyeloablative peripheral blood stem cell transplantation in children and young adults with ultra-high-risk solid tumors. aNK-DLI were CD3(+)-depleted, CD56(+)-selected lymphocytes, cultured for 9 to 11 days with recombinant human IL-15 plus 4-1BBL(+)IL-15Rα(+) artificial antigen-presenting cells. aNK-DLI demonstrated potent killing capacity and displayed high levels of activating receptor expression. Five of 9 transplant recipients experienced acute graft-versus-host disease (GVHD) following aNK-DLI, with grade 4 GVHD observed in 3 subjects. GVHD was more common in matched unrelated donor vs matched sibling donor recipients and was associated with higher donor CD3 chimerism. Given that the T-cell dose was below the threshold required for GVHD in this setting, we conclude that aNK-DLI contributed to the acute GVHD observed, likely by augmenting underlying T-cell alloreactivity. This trial was registered at www.clinicaltrials.gov as #NCT01287104.

Graft-versus-host disease in recipients of grafts from natural killer T cell-deficient (Jalpha281(-/-)) donors

Valpha14i natural killer T cells (NKT cells) are a CD1-restricted subset of NKT cells that express an invariable Valpha14+ Jalpha281+ alphabeta T-cell receptor. To determine whether the absence of Valpha14i NKT cells from the graft affects the development of acute GVHD, we induced GVH reactions using Jalpha281(-/-) mice as donors in the C57BL/6-->(C57BL/6 x DBA/2)F1-hybrid strain combination. Recipients of grafts from Jalpha281(-/-) donors were not protected from either the morbidity or the severe wasting syndrome associated with the development of acute GVHD, but the concentrations of some T helper 1 (Th1) and Th2 cytokines were different from those seen in recipients of grafts from wild-type donors. Interferon-gamma was seen earlier (day 4) in recipients of grafts from Jalpha281(-/-) donors but did not reach the concentrations seen in recipients of grafts from wild-type donors on day 8 (P < 0.02). On day 8, the amount of tumour necrosis factor-alpha released into the serum following the injection of a small amount of lipopolysaccharide was lower in recipients of grafts from Jalpha281(-/-) donors (P < 0.02). The amount of interleukin (IL)-5 was also lower in recipients of grafts from Jalpha281(-/-) donors, when compared to the concentration seen in recipients of grafts from wild-type donors (P < 0.002). IL-13 was seen in recipients of grafts from Jalpha281(-/-) donors but not in recipients of grafts from wild-type donors. Our findings show that the absence of donor Valpha14i NKT cells is associated with lower concentrations of some Th1 cytokines. We also observed higher IL-13 concentrations and lower IL-5 concentrations in recipients of grafts from Jalpha281(-/-) donors indicating a variable effect on Th2 cytokine production.

Amelioration of acute graft-versus-host disease by NKG2A engagement on donor T cells

Acute graft-versus-host disease (aGVHD) remains a major complication of allogeneic bone marrow transplantation, which is caused by donor T cells specific for host alloantigens. In a murine model, we found that donor T cells expressed a natural killer cell inhibitory receptor, CD94/NKG2A, during the course of aGVHD. Administration of an anti-NKG2A mAb markedly inhibited the expansion of donor T cells and ameliorated the aGVHD pathologies. These results suggested that the CD94/NKG2A inhibitory receptor expressed on host-reactive donor T cells can be a novel target for the amelioration of aGVHD.

GVHD-associated enteropathy and endotoxemia in F1-hybrid recipients of NK1.1-depleted grafts

Our previous work using a C57BL/6-->(C57BL/6 x DBA/2)F1-hybrid model of acute GVHD showed that mortality can be completely prevented if grafts are depleted of NK1.1+ cells in vitro. To achieve this protection, it was necessary to inject the donors with polyinosinic:polycytidylic acid 18 h before the graft was harvested. In another study, we showed that interferon (IFN)-gamma production and lipopolysaccharide (LPS)-induced tumour necrosis factor (TNF)-alpha release are markedly reduced in these recipients, suggesting that this treatment abrogates the Th1-mediated immune response that underlies the development of this disease. However, because it has also been hypothesized that cytotoxic NK1.1+ cells mediate injury to tissues targeted by the GVH reaction, we wished to determine whether NK1.1 depletion of the graft would also prevent the development of GVHD-associated enteropathy and endotoxemia. We therefore induced GVH reactions in (C57BL/6 x DBA/2)F1 hybrids using either untreated grafts from unstimulated C57BL/6 donors, or NK1.1-depleted grafts from poly I:C-stimulated donors. We identified intestinal lesions morphologically in sections of ileum collected from each group of recipients but not in control mice. We also compared endotoxin levels in the sera. Our results indicate that GVHD-associated enteropathy occurs in both groups of recipients, and that the levels of LPS in the sera do not differ significantly.

Early intestinal Th1 inflammation and mucosal T cell recruitment during acute graft-versus-host reaction

Little is understood about the earliest cytokine responses and the role(s) of donor CD4 T cells in the intestine during the induced graft-vs-host reaction (GVHR). We investigated the activation and mucosal homing phenotype of the donor CD4 cells and the kinetics of cytokine responses within the intestine and associated lymphoid tissues during early GVHR. Significant frequencies of donor CD4 cells accumulated within recipient Peyer's patches (PP), mesenteric lymph nodes (MLN), lamina propria (LP), and spleen (SP), during the first 9 days of GVHR. Many donor CD4 cells in SP, MLN, and LP expressed CD44 and also expressed de novo the mucosal homing integrin alpha(4)beta(7) (LPAM-1). A large IFN-gamma response occurred by day 3 in cells from PP and MLN, but much later (day 9) in SP and LP cells. IL-10 production by SP and MLN cells was elevated initially but declined substantially by day 9. IL-4 production by SP, MLN, and PP cells was low on day 3 and showed gradual decline in LP by day 9. IL-5 production by LP cells gradually increased in direct contrast to IL-5 production by MLN cells. The MLN CD4 cells showed the most dynamic changes, with high numbers of activated/effector donor CD4 cells and altered cytokine production consistent with a developing Th1 response. The IFN-gamma responses in PP and MLN preceded that of the SP, suggesting an intestinal origin for some Th1 effector cells in GVHR. Donor CD4 T cells apparently acquire the ability to home to the LP during early GVHR.

Acute, lethal graft-versus-host disease in an F1-hybrid model using grafts from parental-strain, T-cell receptor-delta gene knockout donors

Gammadelta T cells have been implicated in the pathogenesis of acute graft-versus-host disease (GVHD). We therefore performed experiments to determine whether mortality from GVHD is reduced in C57BL/6 x DBA/2 F1-hybrid (BDF1-hybrid) mice when parental strain, T-cell receptor-delta (TCRdelta) knockout (KO) donors are used. We compared mortality, weight loss, interferon-gamma (IFN-gamma) production and cytotoxic activity in recipients of either wild-type or TCRdelta KO grafts. In both groups there was significant weight loss and an identical level of mortality. Elevated IFN-gamma levels were present in both groups, but recipients of TCRdelta KO grafts produced twice as much as recipients of wild-type grafts. Elevated natural killer (NK) and NK-like activity was also seen in both. These results demonstrate that TCRdelta KO grafts can induce GVHD as severe as that seen in recipients of wild-type grafts, a finding that is at odds with studies demonstrating reduced mortality when gammadelta T cells are purged from donor mice. We suggest that the inconsistency may lie in the higher levels of IFN-gamma seen with TCRdelta KO grafts and that the protection afforded by the absence of gammadelta T cells in the graft is overwhelmed by the higher levels of IFN-gamma.

Depletion of natural killer cells from the graft reduces interferon-gamma levels and lipopolysaccharide-induced tumor necrosis factor-alpha release in F1 hybrid mice with acute graft-versus-host disease

BACKGROUND

We wished to determine whether removal of NK1.1+ cells from the graft provides protection against acute graft-versus-host disease (GVHD) by obviating the Th1 immune response that underlies the development of this disease.

METHODS

Graft-versus-host (GVH) reactions were induced in two groups of (C57BL/6 x DBA/2)F1 hybrid mice. The first received grafts harvested from polyinosinic:polycytidylic acid-stimulated, C57BL/6 donors and depleted in vitro of NK1.1+ cells. This treatment provides protection against GVHD-associated mortality and cachexia. The second received unmodified grafts. We compared interferon-gamma and interleukin-10 production as well as the levels of engraftment in these two groups. Lipopolysaccharide-induced tumor necrosis factor-alpha (TNF-alpha) release was also compared since TNF-alpha levels in GVH mice following injection of a sublethal dose of endotoxin provide an index of macrophage priming by Th1 cytokines.

RESULTS

Interferon-gamma production was absent in recipients of NK1.1-depleted grafts at the time when high levels were seen in recipients of unmodified grafts. Following lipopolysaccharide injection, high levels of TNF-alpha were observed in recipients of unmodified grafts, whereas negligible amounts were present in recipients of NK1.1-depleted grafts. The use of NK1.1-depleted grafts did not result in a reduced level of engraftment of CD4+ or CD8+ cells.

CONCLUSIONS

These results suggest that NK1.1 depletion of the graft confers protection against mortality by interfering with an immunoregulatory mechanism that results in the development of a Th1 response in GVH mice, and does not result in abortion of the graft. Because macrophage priming is prevented, recipients are also protected from the exaggerated sensitivity to endotoxin seen in mice with acute GVHD.

Suppression of graft-versus-host disease and amplification of graft-versus-tumor effects by activated natural killer cells after allogeneic bone marrow transplantation

Bone marrow transplantation (BMT) is currently used for the treatment of a variety of neoplastic diseases. However, significant obstacles limiting the efficacy of allogeneic BMT are the occurrence of graft-versus-host disease (GvHD) and tumor relapse. Natural killer (NK) cells exert a variety of immunologic and homoeostatic functions. We examined whether adoptive transfer of activated NK cells of donor type would prevent GvHD after allogeneic BMT in mice. Lethally irradiated C57BL/6 (H-2(b)) mice, were transplanted with MHC incompatible BALB/c (H-2(d)) bone marrow cells and spleen cells and rapidly succumbed to acute GvHD. In contrast, mice that also received activated NK cells of donor type exhibited significant increases in survival. In determining the mechanism by which the NK cells prevented GvHD, mice were concurrently treated with a neutralizing antibodies to the immunosuppressive cytokine TGFbeta. Anti-TGFbeta completely abrogated the protective effects of the activated donor NK cells indicating that TGFbeta plays an important role in the prevention of GvHD by NK cells. We then examined whether activated NK cells of donor type after allogeneic BMT would induce graft-versus-tumor (GvT) effects without GvHD in mice bearing a murine colon adenocarcinoma (MCA-38). 10 d after receiving the tumor, in which the mice had demonstrable lung metastases, recipients received an allogeneic BMT with or without activated NK cells. Administration of activated NK cells resulted in significant GvT effects after allogeneic BMT as evidenced by increases in median survival and fewer lung metastasis. No evidence of GVHD was detected compared with recipients receiving spleen cells alone which also developed fewer lung metastases but in which all had succumbed to GVHD. Thus, our findings suggest that adoptive immunotherapy using activated donor NK cells combined with allogeneic BMT inhibits GvHD and promotes GvT in advanced tumor-bearing mice. These results also suggest that GvT and GvHD can be dissociable phenomena.

Graft-versus-host reactions in dermatology

Graft-versus-host reactions frequently produce cutaneous and systemic complications in patients receiving bone marrow transplants. Characteristic skin involvement typically heralds graft-versus-host reactions and significantly contributes to the morbidity associated with marrow transplants. Familiarity with these reactions and their treatment is important to dermatologists involved in the care of marrow transplant recipients.

The potential role of NK cells in the separation of graft-versus-tumor effects from graft-versus-host disease after allogeneic bone marrow transplantation

Allogeneic bone marrow transplantation (BMT) is being increasingly used for the treatment of a variety of cancers ranging from leukemias to breast cancer. However, significant obstacles currently limit the efficacy of this treatment procedure. The predominant two are the occurrence of graft-versus-host disease (GVHD) and relapse from the cancer. While regimens exist that prevent the occurrence or severity of GVHD, these same regimens also increase the rate of relapse. Conversely, most attempts to reduce the relapse rate also result in increased GVHD. The use of NK cells as an adoptive immunotherapy after BMT is attractive for several reasons. NK cells exhibit antitumor effects both in vitro and in animal models and may, therefore, promote graft-versus-tumor (GVT) effects to remove minimal residual disease after allogeneic BMT. NK cells have also been shown to promote hematopoietic engraftment and donor cell reconstitution after allogeneic BMT in mice. The effects of NK cells on hematopoiesis are believed to be due to the hematopoietic growth factors they can produce after activation. Another advantage in using NK cells is that they can prevent the occurrence of GVHD after allogeneic BMT in mice. This effect is mediated at least in part by the immunosuppressive cytokine, transforming growth factor beta (TGF-beta). BMT studies in mice also indicate that the beneficial effects of NK cells are optimal if they are administered soon after the transplant. Thereafter, NK cells and, more importantly, IL-2, which is used to activate them, are detrimental and can exacerbate the subsequent GVHD. Thus, the use of activated NK cells after allogeneic BMT may provide GVT effects without inducing GVHD.

Graft-versus-leukemia effect and its clinical implications

Graft-versus-leukemia (GVL) effect is an immunologically important phenomenon which decreases the relapse rate of leukemia after allogeneic bone marrow transplantation. GVL effect is sometimes associated with the occurrence of graft-versus-host disease (GVHD). Analyses of GVL effect and GVHD showed that these two phenomena were separable in some conditions. Although we cannot yet completely control the development of the GVL effect without inducing GVHD in humans, basic analyses using animal models show potential benefits of the GVL effect for clinical applications. Autologous GVHD is another important phenomenon which can help to eradicate minimal residual disease. Interleukin 2 and/or cyclosporin A are extensively used in animal models and in humans to induce autologous GVHD, showing beneficial effects. In the future, cytokine usage and allogeneic stem cell transplantation or leukocyte infusion appear to be promising in the control of minimal residual disease. Further studies on the mechanisms of GVL effects and GVHD may well open a new era for cell transplantation.

Macrophage priming and lipopolysaccharide-triggered release of tumor necrosis factor alpha during graft-versus-host disease

In this report we have investigated macrophage (M phi) activity and tumor necrosis factor alpha (TNF-alpha) production during graft-vs.-host disease (GVHD). TNF-alpha production by M phi requires two signals: priming of M phi by interferon followed by triggering of TNF-alpha production and release by lipopolysaccharide (LPS). The state of M phi activation was examined in nonirradiated B6AF1 recipient mice injected with either 60 x 10(6) (acute GVHD) or 30 x 10(6) (nonlethal GVHD) parental B6 lymphoid cells. During the early phase of acute GVHD, administration of normally sublethal amounts of LPS-triggered release of significant amounts of TNF-alpha into the serum resulting in death of the animals within 36 h. Normal animals treated with the same dose of LPS neither died nor produced detectable amounts of serum TNF-alpha. In vitro studies demonstrated that M phi were primed during GVHD. The level of M phi priming was greater during acute GVHD than nonlethal GVHD since 100-fold less LPS was required to trigger killing of a TNF-alpha-sensitive cell line by M phi from acute GVHD animals. The amount of TNF-alpha released into the serum after LPS injection increased during the course of the GVHD and was significantly greater in acute GVH-reactive mice. Endogenous LPS was detected in the serum of acute GVH-reactive animals coincident with the onset of mortality. The data provide evidence that during GVHD M phi are primed as a result of the allogeneic reaction and that endogenous LPS therefore triggers M phi production of TNF-alpha resulting in the symptoms characteristic of acute GVHD.

Identification of natural killer (NK) cells in lesions of human cutaneous graft-versus-host disease: expression of a novel NK-associated surface antigen (Kp43) in mononuclear infiltrates

We performed an immunohistochemical analysis of skin biopsies from 13 allogeneic bone marrow transplant (BMT) recipients, undergoing either acute graft-versus-host-disease (aGVHD, n = 8) or chronic GVHD (cGVHD, n = 5). A panel of different monoclonal antibodies (MoAb) was employed including anti-CD2, -CD3, -CD4, -CD8, -CD11b, -CD16, -CD56, and -CD57, as well as a recently described reagent (HP-3B1) specific for a novel natural killer (NK)-associated cell-surface antigen (Kp43). Our data indicate that in aGVHD lesions the proportions of CD2+ cells often exceeded those detected with anti-CD3 MoAb. Double labeling confirmed the presence of CD2+ CD3- lymphocytes and suggested the coexpression in some cells of CD2 and CD11b. When MoAb specific for non-lineage-restricted NK-associated markers were employed, anti-CD56 and -CD57 occasionally stained variable numbers of lymphocytes (means = 14.6% of mononuclear cells in 0.05 mm2, range less than 1-48% and means = 10.3%, range 2-25%, respectively), whereas no CD16+ lymphocytes were observed. In contrast, most samples consistently displayed substantial proportions of Kp43+ cells (means = 32.8%, range 12-63%), which appeared CD3- and were mainly located at the dermoepidermal junction. On the other hand, sections from most (four of five) cGVHD lichenoid lesions analyzed displayed lower proportions of Kp43+ and CD56+ cells. Our data point out the interest of the anti-Kp43 MoAb to identify NK cells in aGVHD lesions, suggesting their pathogenetic participation.

Natural suppressor-like cells in local graft-vs-host disease

We examined the generation of suppressor cells in the popliteal lymph nodes (PLN) of F1 recipients of parental spleen cells in rats, i.e., a local form of graft-vs-host disease (GVHD). PLN cells of F1 recipients, or subpopulations of these cells, were tested for their ability to suppress the mixed lymphocyte culture and cell-mediated lympholysis. Suppressor cells were consistently generated in this local GVHD reaction. In contrast to others, we found no evidence that suppression was mediated by T cells. Instead, the suppressor cells had the CD5-, CD8+, asialo-GM1+ phenotype characteristic of rat NK cells. The suppression exerted by these cells was nonspecific and was not mediated by a veto effect. The suppressor cells did not kill alloreactive T cell blasts, but had strong NK activity. These cells appear to be of donor origin. These local-GVHD-associated NK-like suppressor cells are similar, but not identical, to the natural suppressor cells described by other investigators in various experimental systems.

Biology of natural killer cells

Studies of cytotoxicity by human lymphocytes revealed not only that both allogeneic and syngeneic tumor cells were lysed in a non-MHC-restricted fashion, but also that lymphocytes from normal donors were often cytotoxic. Lymphocytes from any healthy donor, as well as peripheral blood and spleen lymphocytes from several experimental animals, in the absence of known or deliberate sensitization, were found to be spontaneously cytotoxic in vitro for some normal fresh cells, most cultured cell lines, immature hematopoietic cells, and tumor cells. This type of nonadaptive, non-MHC-restricted cellmediated cytotoxicity was defined as “natural” cytotoxicity, and the effector cells mediating natural cytotoxicity were functionally defined as natural killer (NK) cells. The existence of NK cells has prompted a reinterpretation of both the studies of specific cytotoxicity against spontaneous human tumors and the theory of immune surveillance, at least in its most restrictive interpretation. Unlike cytotoxic T cells, NK cells cannot be demonstrated to have clonally distributed specificity, restriction for MHC products at the target cell surface, or immunological memory. NK cells cannot yet be formally assigned to a single lineage based on the definitive identification of a stem cell, a distinct anatomical location of maturation, or unique genotypic rearrangements.

Association between the degree of thymic dysplasia and the kinetics of thymic NK cell activity during the graft-versus-host reaction

In this study, by employing different cell doses and parent into F1 hybrid combinations, we have investigated the relationship between the severity of thymic medullary dysplasia and the kinetics of thymic natural killer (NK) cell activity after the induction of graft-versus-host (GVH) reactions. GVH reactions were induced by injecting different doses (30, 20, 10 X 10(6] of C57BL/6 (B6) of A parental lymphoid cells (PLC) into non-X-irradiated adult B6xAF1 (B6AF1) mice. On different days after the induction of GVH reactions, the thymuses were examined histologically and thymocyte NK cell activity was tested by using YAC targets. Our results show that, depending upon the genotype and dose of PLC injected, various degrees of thymic medullary dysplasia (mild, moderate, or severe) can be induced. Furthermore, severe to moderate thymic medullary dysplasia is observed only in those groups of GVH-reactive mice in which thymic NK cell activity occurs early and increases rapidly. In contrast, when mild thymic medullary dysplasia or no thymic alterations was observed, thymic NK cell activity peaked later and was of lower intensity than that of the groups with moderate to severe lesions. These results suggest an association between the degree of thymic medullary dysplasia and the kinetics of NK cell activity in the thymus. Furthermore, the different degrees of thymic medullary dysplasia as described here may serve as a powerful tool to study the role of thymic medullary dysplasia in determining the duration of T-cell immunodeficiency associated with the GVH reactions.