Short-term changes in frequencies of circulating leukocytes associated with narrowband UVB phototherapy in people with clinically isolated syndrome.

Clinically isolated syndrome (CIS) is the earliest clinical episode in multiple sclerosis (MS). Low environmental exposure to UV radiation is implicated in risk of developing MS, and therefore, narrowband UVB phototherapy might delay progression to MS in people with CIS. Twenty individuals with CIS were recruited, and half were randomised to receive 24 sessions of narrowband UVB phototherapy over a period of 8 weeks. Here, the effects of narrowband UVB phototherapy on the frequencies of circulating immune cells and immunoglobulin levels after phototherapy are reported. Peripheral blood samples for all participants were collected at baseline, and 1, 2, 3, 6 and 12 months after enrolment. An extensive panel of leukocyte populations, including subsets of T cells, B cells, monocytes, dendritic cells, and natural killer cells were examined in phototherapy-treated and control participants, and immunoglobulin levels measured in serum. There were significant short-term increases in the frequency of naïve B cells, intermediate monocytes, and fraction III FoxP3+ T regulatory cells, and decreases in switched memory B cells and classical monocytes in phototherapy-treated individuals. Since B cells are increasingly targeted by MS therapies, the effects of narrowband UVB phototherapy in people with MS should be investigated further.

TLR9 mediated regulatory B10 cell amplification following sub-total body irradiation: Implications in attenuating EAE.

Autoimmunity and inflammation are controlled in part by regulatory B (Breg) cells, including the recently identified IL-10-competent B10 cell subset that represents 1%-3% of mouse spleen B cells. In this study, the influence of irradiation on Breg/B10 cell generation and IL-10 production mediated by TLR9 signaling pathways was investigated. Spleen and peritoneal cavity Breg/B10 cell frequencies were significantly expanded three weeks after sub-total body irradiation (sub-TBI, 5Gy or 10Gy) in adult male wild type (WT) C57BL/6(B6) mice but not in TLR9-/- mice. TLR9 agonist ODN1826 stimulation in vitro for 5h induced more B10 cells to express cytoplasmic IL-10 in sub-TBI WT mice than in TLR9-/- mice. Prolonged ODN1826 stimulation (48h) induced additional spleen CD19hiCD5+CD1dhi B cells to express IL-10. TLR9-dependent signaling molecules, MyD88, TRAF6 and IRF8 are involved in sub-TBI induced Breg/B10 cells development and expansion. Furthermore, using a mouse model for multiple sclerosis, we show here that sub-TBI induced Breg/B10 cells dramatically inhibit disease onset and severity when transferred into mice with established experimental autoimmune encephalomyelitis (EAE). Adoptively transferred sub-TBI induced Breg cells significantly suppress inflammatory T cell responses of TH17 and TH1 types in EAE mice. In conclusion, sub-TBI can drive Breg/B10 cell development and expansion, which could be used as a novel tool for suppressing undesirable immunity. The ex vivo expansion and reinfusion of autologous Breg/B10 cells may provide a novel and effective in vivo treatment for severe autoimmune diseases that are resistant to current therapies.

Assessment of selected B cells populations in the workers of X-ray departments.

OBJECTIVES: Workers of X-ray departments are occupationally exposed to long-term low levels of ionizing radiation (LLIR), which may affect their humoral immunity. The aim of the study was to assess the influence of LLIR on the number and proportion of B cells (CD19+), B1 cells (CD5+CD19+) and memory B cells (CD27+CD19+) in peripheral blood of such workers. MATERIALS AND METHODS: In the study group of 47 X-ray departments workers and the control group consisting of 38 persons, the number and percentage of CD19+, CD5+CD19+, CD27+CD19+ cells as well as CD5+CD19+/CD19+ and CD27+CD19+/CD19+ cell ratios were assessed using flow cytometry. Additionally, the study group was divided into 2 groups by the length of employment below and over 15 years and analysis adjusted for age and smoking habit was performed. RESULTS: The total number of CD19+ cells showed significant increase in the group of workers in comparison with the persons from the control group, whereas the percentage of CD5+CD19+ cells as well as CD27+CD19+/CD19+ and CD5+CD19+/CD19+ cell ratios were lower. Percentage, number of CD5+CD19+ cells and CD5+CD19+/CD19+ cell ratio were significantly lower in the workers with length of employment longer than 15 years in comparison with those employed below 15 years. Moreover, we found positive associations between the number of CD19+ cells and employment as well as smoking habit, whereas the number of CD5+CD19+ cells was positively associated with cigarette smoking alone. Percentage of CD5+CD19+ cells as well as CD5+CD19+/CD19+ and CD27+CD19+/CD19+ cell ratios were negatively correlated with employment. CONCLUSIONS: The study suggests association between the suppressive influence of low level ionizing radiation on circulating in peripheral blood, especially of B1 cells as well as of memory B cells, in workers of X-ray units, which is adverse in relation to microbiological threat.

Possible pitfalls investigating cell death responses in genetically engineered mouse models and derived cell lines.

Genetically engineered mouse models are frequently used to identify pathophysiological consequences of deregulated cell death. Targeting pro-apoptotic or anti-apoptotic proteins of the extrinsic or intrinsic apoptotic signalling cascade is state of the art since more than two decades. Such animal models have been increasingly made use of over the past years to study loss- or gain-of-function consequences of one or more components of the molecular machinery leading to cell death. These studies have helped to separate redundant from non-redundant functions of apoptosis-related proteins in normal physiology and sometimes unravelled unexpected phenotypes. However, correct interpretation of data derived from knockout mice or derived cells and cell lines is often flawed by the comparison of cells originating from different inbred or mixed genetic backgrounds. Here we want to highlight some basic problems associated with genetic background-based modulation of cell death sensitivity and describe some methods that we use to investigate cell death responses in hematopoietic and non-hematopoietic cells. Thereby, we show that hematopoietic cells derived from wild type mice on a C57BL/6:129/SvJ recombinant mixed genetic background are significantly more resistant to spontaneous cell death or DNA-damage induced apoptosis in vitro than cells derived from inbred C57BL/6 mice. Furthermore, we show as an example that C57BL/6 mice are more susceptible to γ-irradiation induced cell death after whole body irradiation in vivo and subsequent T cell lymphomagenesis.

Lymphocyte subsets and their H2AX phosphorylation in response to in vivo irradiation in rats.

PURPOSE: The objective of the study was to investigate differences in the radiosensitivity of rat peripheral blood lymphocyte subsets identified by expression of surface clusters of differentiation markers (CD3, CD4, CD8, CD45RA, CD161) after whole-body in vivo gamma-ray irradiation and to assess their individual histone H2AX phosphorylation as an early cell response to irradiation. MATERIALS AND METHODS: The relative representations of CD45RA B-lymphocytes, CD161 natural killer cells (NK cells), CD3CD4 T-lymphocyte subset and CD3CD8 T-lymphocyte subset in the rat peripheral blood were studied 24-72 hours after irradiation in a dose range of 0-5 Gy. Their intracellular H2AX phosphorylation (γ-H2AX) after 4 Gy and 9 Gy whole-body in vivo irradiation was assessed by multicolour flow cytometry. RESULTS: We determined the linear dose response of radioresistant CD161 NK cells (24 h), both radiosensitive T-lymphocyte subsets (24 h) and CD45RA B-lymphocytes (72 h) after in vivo irradiation. CD45RA B-lymphocytes showed the highest radiosensitivity and we observed pronounced H2AX phosphorylation which remained expressed in these cells for over 4 h after irradiation. CONCLUSION: The combination of the surface immunophenotyping together with intracellular detection of γ-H2AX offers the possibility to assess the absorbed dose of ionizing irradiation with high sensitivity post irradiation and could be successfully applied to biodosimetry.

Immediate and short-, mid- and long-term effects of in vivo ionizing radiation exposure in BALB/c mice: I. Activation of lymphocytes and subpopulations.

BACKGROUND: The main aim of this work was to study the effects of single whole-body irradiation (WBI) on the lymphoproliferative response in radiation-sensitive BALB/c mice. MATERIALS AND METHODS: Mice were irradiated (0-5 Gy) and euthanized immediately afterward; other animals were subjected to WBI (4 Gy) and were analyzed after periods of 0-180 days. Splenic cell number, lymphoproliferative response and lymphocyte subpopulations were studied. RESULTS: This study shows that immediately after exposure, an inhibition of the basal mitogen lymphoproliferative response was produced; furthermore, B-cells appear to be more radiosensitive than T-cells. However, up to 90 day's post-irradiation, mice spleens clearly show low cell numbers, and subpopulations and T-cell mitogens did not return to normal, while the basal response and B-cell mitogens peaked on day 15 post-WBI. CONCLUSION: In our model, B-cells regenerate earlier than T-cells, while Th lymphocytes regenerate faster than Tc lymphocytes.

In vivo sensitized and in vitro activated B cells mediate tumor regression in cancer adoptive immunotherapy.

Adoptive cellular immunotherapy utilizing tumor-reactive T cells has proven to be a promising strategy for cancer treatment. However, we hypothesize that successful treatment strategies will have to appropriately stimulate not only cellular immunity, but also humoral immunity. We previously reported that B cells in tumor-draining lymph nodes (TDLNs) may function as APCs. In this study, we identified TDLN B cells as effector cells in an adoptive immunotherapy model. In vivo primed and in vitro activated TDLN B cells alone mediated effective (p < 0.05) tumor regression after adoptive transfer into two histologically distinct murine pulmonary metastatic tumor models. Prior lymphodepletion of the host with either chemotherapy or whole-body irradiation augmented the therapeutic efficacy of the adoptively transferred TDLN B cells in the treatment of s.c. tumors as well as metastatic pulmonary tumors. Furthermore, B cell plus T cell transfers resulted in substantially more efficient antitumor responses than B cells or T cells alone (p < 0.05). Activated TDLN B cells conferred strong humoral responses to tumor. This was evident by the production of IgM, IgG, and IgG2b, which bound specifically to tumor cells and led to specific tumor cell lysis in the presence of complement. Collectively, these data indicate that in vivo primed and in vitro activated B cells can be employed as effector cells for cancer therapy. The synergistic antitumor efficacy of cotransferred activated B effector cells and T effector cells represents a novel approach for cancer adoptive immunotherapy.

Chemical induction of the bystander effect in normal human lymphoblastoid cells.

Many studies investigating the bystander effect have used ionizing radiation to evaluate this phenomenon, whereas very few have determined whether genotoxic chemicals are also capable of inducing this effect. Here, we show that two such chemicals, mitomycin C, a bifunctional alkylating agent and phleomycin, a glycopeptide antibiotic of the bleomycin family, cause normal human B lymphoblastoid cells to produce media soluble factors that induce a bystander effect in unexposed cells. Ionizing radiation was used in parallel experiments to verify the existence of the bystander effect in these cells. Micronuclei in Cytochalasin B-blocked binucleated cells were used as the endpoint. Conditioned media obtained from cells exposed to mitomycin C induced a 1.5-3 fold increase, while conditioned media from phleomycin induced a 1.5-4 fold increase, and conditioned media from irradiated cells induced a 2-8 fold increase in micronuclei. We conclude that the bystander effect is not restricted to ionizing radiation, suggesting it may be a part of a general cellular stress response.

The effect of in vitro ã-irradiation on mitogenic responsiveness of murine lymphocytes

The objective of this study was to analyze the proliferative response of BALB/cmice lymphocytes after in vitro irradiation (0.05 to 6 Gy). The capability of irradiatedlymphocytes for proliferating without any stimulation and after activation withspecific T and B cell mitogens has been evaluated. The results show that ionizingradiation significantly inhibits spontaneous cellular proliferation and that induced bymitogens and that variations in the degree of inhibition are found depending on theinducing proliferation mitogens and the dosage applied. The conclusion drawn is thatdifferent lymphocyte populations have different radiosensitivities, being B cells moresensitive to ionizing irradiation than T cells. Besides, the effects of gamma-irradiationvary according to the different subpopulations of T cells or, alternatively, to differentT-dependent activation mechanisms (AU)

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The effect of in vitro gamma-irradiation on mitogenic responsiveness of murine lymphocytes.

The objective of this study was to analyze the proliferative response of BALB/c mice lymphocytes after in vitro irradiation (0.05 to 6 Gy). The capability of irradiated lymphocytes for proliferating without any stimulation and after activation with specific T and B cell mitogens has been evaluated. The results show that ionizing radiation significantly inhibits spontaneous cellular proliferation and that induced by mitogens and that variations in the degree of inhibition are found depending on the inducing proliferation mitogens and the dosage applied. The conclusion drawn is that different lymphocyte populations have different radiosensitivities, being B cells more sensitive to ionizing irradiation than T cells. Besides, the effects of gamma-irradiation vary according to the different subpopulations of T cells or, alternatively, to different T-dependent activation mechanisms.

Role of distinct immune components in the radiation-induced abrogation of systemic lupus erythematosus development in mice.

The New Zealand Black x New Zealand White F1 [(NZB/NZW) F1] mouse develops an autoimmune condition resembling aspects of human systemic lupus erythematosus (SLE). We investigated the effects of a novel prophylactic thoraco-abdominal gamma irradiation protocol on the onset and evolution of lupus in these animals. Survival of irradiated mice was higher when compared with nonirradiated mice. Kidney lesions were milder and autoantibody levels were lower in irradiated mice. To identify possible mechanisms involved in the radiation-induced improvement of disease, distinct components of humoral and cellular immune responses were evaluated. Because B-1 cells are known to be involved in various autoimmune diseases, we investigated the participation of these cells in SLE progression. Unexpectedly, B-1 cells were not depleted in (NZB/NZW) F1, even after several rounds of irradiation. No alterations were found in viability and physiology of B-1 cells in SLE animals with the exception of constitutive overexpression of the anti-apoptotic molecule Bcl-2, which may account for the observed radioresistance. Thus, a role for B-1 cells in murine SLE cannot be excluded, since the irradiation protocol did not effectively eliminate these cells. Additionally, we demonstrate a marked delay in the ability of splenocytes to repopulate the spleen after irradiation in (NZB/NZW) F1, in contrast to leucocytes in other cellular compartments. The implications of these findings for the fate of SLE in this model are discussed.

Cutting edge: inherent and acquired resistance to radiation-induced apoptosis in B cells: a pivotal role for STAT3.

Radiation-induced apoptosis (RiA) is used therapeutically for tumor cell ablation as well as a tool to characterize hemopoietic cell lineages. We report that the peritoneal B-1 B cell subset is selectively resistant to RiA. Inherent radioresistance is not shared by splenic B-2 or B-1 cells. However, it is conferred upon B-2 cells by BCR crosslinking in the presence of IL-6 or IL-10. In vivo experiments with gene-targeted mice confirm that IL-6 and, to a lesser extent, IL-10 are the relevant stimuli that combine with BCR ligands to promote B-1 cell radioresistance. STAT3 promotes cell survival in response to selected growth factors, and is activated by combined BCR crosslinking and IL-6 (IL-10). Importantly, STAT3(-/-) B-1 cells become susceptible to irradiation, indicating that STAT3 activation by the BCR in the presence of IL costimuli account for the inherent radioresistance of peritoneal B-1 B cells.

Several genes contribute to the production of autoreactive B and T cells in the murine lupus susceptibility locus Sle1c.

The systemic lupus erythematosus 1 (Sle1) locus mediates the loss of tolerance to nuclear Ags in the NZM2410 mouse model of lupus through intrinsic defects in both B and T cells. Congenic analysis has shown that Sle1 corresponds to at least three genetic loci, Sle1a, Sle1b, and Sle1c. Telomeric Sle1c is associated with abnormal B cell responses to subthreshold stimulation with anti-IgM and C3d and with decreased T-dependent humoral immune responses. We have proposed that these phenotypes resulted from polymorphisms in the C3 complement receptor Cr2 gene. We have also found that Sle1c was associated with the production of histone-specific autoreactive CD4(+) T cells, which correlated with higher activation and proliferative responses, and a reduction in the CD4(+)CD25(+)CD62L(+)forkhead/winged helix transcription factor gene (Foxp3(+)) compartment. In this study we showed, using congenic recombinants, that the decreased humoral immune response and impaired GC formation map to the NZM2410 Cr2 allele. A chronic graft-vs-host disease model also showed that Sle1c produces significantly more autoreactive B cells than B6 controls, and that this phenotype maps to two regions excluding the Cr2 gene. Mixed bone marrow chimera demonstrated that the increased activation, proliferative response, and reduced regulatory T cell compartment were intrinsic to Sle1c-expressing CD4(+) T cells. These phenotypes mapped to the same two loci identified with the chronic graft-vs-host disease model, excluding the Cr2 region. Overall, these results show that Sle1c results in the production of autoreactive B and T cells through the expression of three different genes, one of which is consistent with Cr2, based on the phenotypes of the Cr2-deficient mice, and the other two corresponding to as yet unidentified genes.

The role of host CD4 T cells in the pathogenesis of the chronic graft-versus-host model of systemic lupus erythematosus.

Systemic lupus erythematosus is characterized by production of autoantibodies and glomerulonephritis. The murine chronic graft-vs-host (cGVH) model of systemic lupus erythematosus is induced by allorecognition of foreign MHC class II determinants. Previous studies have shown that cGVH could not be induced in CD4 knockout (CD4KO) mice. We have further explored the role of host CD4 T cells in this model. Our studies now show that B cells in CD4KO mice have intrinsic defects that prevent them from responding to allohelp. In addition, B cells in CD4KO mice showed phenotypic differences compared with congeneic C57BL/6 B cells, indicating some degree of in vivo activation and increased numbers of cells bearing a marginal zone B cell phenotype. The transfer of syngeneic CD4 T cells at the time of initiation of cGVH did not correct these B cell abnormalities; however, if CD4 T cells were transferred during the development and maturation of B cells, then the B cells from CD4KO mice acquire the ability to respond in cGVH. These studies clearly indicate that B cells need to coexist with CD4 T cells early in their development to develop full susceptibility to alloactivation signals.

Postgestational lymphotoxin/lymphotoxin beta receptor interactions are essential for the presence of intestinal B lymphocytes.

Lymphotoxin (LT), a cytokine belonging to the TNF family, has established roles in the formation of secondary lymphoid structures and in the compartmentalization of T and B lymphocyte areas of the spleen. In this study, we examine the role of LT in directing the composition of intestinal lymphocytes. We report that mice deficient in LT have a normal composition of intestinal lamina propria (LP) T lymphocytes, and an absence of intestinal LP B lymphocytes. We further refine this observation to demonstrate that the interaction of LT with the LTbetaR is essential for the presence LP B lymphocytes. The LT/LTbetaR-dependent events relevant for the presence of LP B lymphocytes occur after birth, do not require the presence of Peyer's patches, lymph nodes, or the spleen; and therefore, are distinct and independent from the previously identified roles of LT/LTbetaR. The LT-dependent signal relevant for the presence of LP B lymphocytes is optimally supplied by a LT-sufficient B lymphocyte, and requires a LTbetaR-sufficient radio-resistant, non-bone marrow-derived cell. Based upon the severity of the deficit of LP B lymphocytes we observed, these novel LT/LTbetaR-dependent events are of primary importance in directing the entry and residence of LP B lymphocytes.

The effects of prolonged administration of 5-bromodeoxyuridine on cells of the immune system.

We have determined the in vivo effect of 5-bromodeoxyuridine (BrdU) administered to mice in the drinking water for various lengths of time on the performance of T and B lymphocytes in a number of experimental protocols. Young mice continuously exposed to BrdU fail to gain weight, and the lymphocytes recovered after a prolonged period of exposure are fewer in number than in control mice. The recovery of normal levels of T and B lymphocytes after irradiation is severely impaired. Ag-specific cells responding to Ag in an adoptive transfer model fail to expand as much in the presence of BrdU as in the absence, and the Ag-specific effectors produced in the presence of BrdU are less able to secrete cytokines upon restimulation in vitro. Polarized populations of Tc1 and Tc2 effectors generated in vitro proliferate less in the presence of BrdU, and the resulting effectors make less cytokines per cell upon restimulation. Thus, the incorporation of BrdU into T or B lymphocytes can, under some circumstances, seriously impair the performance of the labeled cells, and these findings raise a note of caution in the interpretation of studies that make use of long-term exposure to BrdU.

Flow cytometry measurements of subsets of T, B and NK cells in peripheral blood lymphocytes of atomic bomb survivors.

Previous studies of blood cells from atomic bomb survivors have shown that frequencies of chromosome aberrations and somatic mutations are elevated in heavily exposed survivors and that T-cell functions and the number of mature T cells are decreased in the survivors who were exposed to radiation as adults. Current progress in flow cytometry allows a sophisticated analysis of various subsets of T, B and NK cells. In the present study, proportions of such subsets in peripheral blood lymphocytes from atomic bomb survivors (159 survivors estimated to be exposed to > or =1.5 Gy) and 252 controls were measured using multiple combinations of monoclonal antibodies to lymphocyte differentiation antigens to investigate whether the previous radiation exposure had altered the composition of the subsets. Among T-cell subsets, the proportion of CD4+ T-cell subsets was decreased significantly in the heavily exposed survivors; this tendency was apparent for the CD4+CD45RA+ naive T-cell subset. However, there were no significant differences in the proportions of CD8+ T-cell subsets between the exposed survivors and controls. As for the B-cell subsets, the proportion of both CD5+ and CD5 B cells as well as CD23+ and CD23- B cells increased in the heavily exposed survivors. Further, no effect of radiation was found in the proportion of NK-cell subsets. These results strongly suggest that previous radiation exposure altered the composition of T and B cells in the peripheral blood of atomic bomb survivors, and they raise the possibility that atomic bomb radiation may have affected the developmental processes of T and B cells.

Chronic ethanol exposure alters leukocyte subsets in repopulating spleens, but does not alter negative selection in thymuses of sublethally irradiated mice.

Results from previous in vitro experiments in this laboratory suggested that ethanol may affect selection processes in the thymus. To determine whether ethanol allows escape of potentially autoreactive T-cell clones from negative selection, we fed ethanol to sublethally irradiated, young, adult C57BR mice during the time of thymic and splenic repopulation as a new model of human third trimester fetal alcohol exposure. The mice received a whole-body, sublethal dose (6 Gy) of gamma irradiation at 5 to 6 weeks of age. Feeding of a liquid diet providing 25% of calories as ethanol (EDC) or an isocaloric control liquid diet was begun 3 days after irradiation and was continued for 5 weeks. Each EDC mouse had 2 weight- and age-matched controls, 1 pair-fed (PF), and 1 fed ad libitum (AD LIB). Average blood alcohol concentrations (90 to 440 mg/100 ml) were higher than those reported previously for neonatal mice exposed to ethanol through lactation. At 5 weeks after irradiation, the EDC mice had lower total thymocyte numbers (p < 0.05) and a higher proportion of CD4-CD8-thymocytes than either the PF or AD LIB mice (p < 0.05), which is consistent with findings using in utero models of ethanol exposure. Ethanol exposure also altered the proportion of leukocyte subsets in repopulating spleens. B cells were the most sensitive to the detrimental effects of ethanol and, as a percentage of total nucleated cells in the spleen, B cells were decreased in the EDC group, compared with both the PF and AD LIB groups (p < 0.05). C57BR mice normally delete by negative selection thymocytes bearing v beta 17 T-cell receptors. There was no discernible effect of ethanol exposure during thymic and splenic repopulation on the expression of V beta 17a on thymocytes and splenic T lymphocytes, indicating that ethanol does not affect negative selection.

Multicolor flow cytometry analysis of blood cell subsets in patients given total body irradiation before bone marrow transplantation.

PURPOSE: Bone marrow transplantation has often been closely linked with accidental or intentional therapeutical irradiation. In both situations, study of the radiosensitivity of human blood cell subsets is of interest. Using one-color flow cytometry analysis of B lymphocytes, T cell subsets, and natural killer cells, we previously reported that lymphocyte subsets exhibit equal radiosensitivity. Taking advantage of recent developments in the knowledge of leukocyte differentiation antigens and flow cytometry technology we undertook a study of blood cell subsets to search for rare populations exhibiting different radiosensitivity. METHODS AND MATERIALS: Thirty patients, who were delivered a 12 Gy fractionated total body irradiation as part of their conditioning regimen before transplantation for malignant disorders, were studied using multicolor flow cytometry. RESULTS: T and B lymphocytes showed a sharp, radiation-induced decrease, with the B lymphocytes (cluster of differentiation (CD) 19+) being the most sensitive. When analyzed by multicolor flow cytometry, all major lymphocyte subsets appeared equally sensitive to the in vivo irradiation; that is, CD3+4+45RO+, CD3+4+45RA+, CD3+4+8-, CD3+4-8+. Therefore, all major lymphocyte subsets sharing the helper phenotype (naive or memory) and the cytotoxic phenotype appeared equally sensitive to in vivo whole body irradiation. In parallel, the CD34+ cell subset remained basically unchanged after whole body irradiation. Finally, the CD3-, 56+, 16+ natural killer cell subset was relatively radioresistant (91 and 74% of its initial value, after 2 and 4 Gy, respectively) as compared to other lymphocyte subsets. CONCLUSION: Our study provides evidence that T and B cell subsets seem to be highly radiosensitive in vivo. The CD34+ progenitor/stem cells and NK cells seem to be more radioresistant. This latter result might provide clues to the understanding of the pathophysiogeny of radiation-induced aplasia and of the engrafment/rejection process following bone marrow transplantation.