Toxicity of high-activity 111In-Octreotide therapy in patients with disseminated neuroendocrine tumours.

Disseminated neuroendocrine tumours are difficult to treat and are generally not responsive to radiotherapy or chemotherapy. Nuclear medicine techniques using a radiolabelled somatostatin analogue, 111In-Octreotide, have been used for the diagnosis of neuroendocrine tumours. It has been suggested that high activities of such an agent may have a therapeutic effect. The aims of this study were to assess toxicity and to determine if there had been evidence of efficacy. Eight patients with known disseminated neuroendocrine tumours were enrolled in the study; six had carcinoid tumours, one had a medullary cell carcinoma of the thyroid and one patient had a malignant gastrinoma. Between 1.3 and 4.6 GBq of 111In-Octreotide were administered to each patient for up to five administrations over 12 months. A total of 23 administrations were given. Tests of vital signs, renal, liver and endocrine function as well as haematological markers were taken before and after treatment. The treatment was well tolerated with only one patient suffering from a sensation of flushing during the infusion but no changes in vital sings. There was a transient (up to 48 h) drop in circulating lymphocytes in four patients and platelets in two patients; no supportive therapy was needed. One patient with severe renal impairment had a slight reduction in glomerular filtration rate. We conclude that high-activity 111In-Octreotide is well tolerated with low toxicity and can be considered for use in patients with disseminated neuroendocrine tumours. Further work is now being performed to assess efficacy.

Bronchoalveolar lavage in bronchiolitis obliterans organizing pneumonia primed by radiation therapy to the breast.

BACKGROUND: Growing evidence indicates that unilateral lung irradiation for breast cancer may "prime" the development of migratory lung infiltrates with histologic features of bronchiolitis obliterans organizing pneumonia. OBJECTIVE: Our purpose was to evaluate the cytologic and immunocytologic features of bronchoalveolar lavage in this condition. METHODS: We analyzed the profile bronchoalveolar lavage cell differentials and lymphocyte subpopulations of 11 women with bronchiolitis obliterans organizing pneumonia syndrome after radiation therapy for breast cancer in comparison to 9 healthy women. RESULTS: The bronchoalveolar lavage analysis demonstrated a significant increase in the percentage of lymphocytes (36.7% +/- 5.4% vs 8.6% +/- 1.1%, P =.0002), neutrophils (3.8% +/- 1.2% vs 0.6% +/- 0.2%, P =.005), eosinophils (2.4% +/- 1% vs 0.3% +/- 0.1%, P =.01), and mast cells (1.4% +/- 0.6% vs 0.1% +/- 0.02%, P =.05) with a significant decrease in the percentage of macrophages (56.1% +/- 6% vs 90.3% +/- 1.4%, P =.0002) in patients with bronchiolitis obliterans organizing pneumonia compared with the control subjects. The percentage of CD3(+) cells was significantly increased in patients with bronchiolitis obliterans organizing pneumonia (93.7% +/- 1.3% vs 70.9% +/- 4%, P =.0004), with a significant decrease in CD4(+) cells (32.7% +/- 4.7% vs 55.4% +/- 2. 6%, P =.002) and a significant increase in CD8(+) cells (61.2% +/- 4. 8% vs 37.5% +/- 2.9%, P =.003) in comparison to control subjects. The CD4/CD8 ratio was significantly reduced in patients with bronchiolitis obliterans organizing pneumonia compared with control subjects (0.6% +/- 0.1% vs 1.5% +/- 0.1%, P =.001). CONCLUSION: These data add to the view that unilateral lung irradiation for breast cancer may "prime" the development of a syndrome quite similar to idiopathic bronchiolitis obliterans organizing pneumonia.

Occupational exposure to high frequency electromagnetic fields and its effect on human immune parameters.

The present study recorded a considerable excess of recommended exposure limits in the vicinity of shortwave diathermy devices used for medical treatment of patients. Different kinds of field probes were used to measure electric and magnetic field strength and the whole body exposure of medical personnel operating shortwave, decimeter wave and microwave units was calculated. To investigate the influence of chronic exposure on the immune system of operators, blood was sampled from physiotherapists working at the above mentioned devices. Eighteen exposed and thirteen control persons, matched by sex and age, were examined. Total leucocyte and lymphocyte counts were performed and leucocytic subpopulations determined by flow cytometry and monoclonal antibodies against surface antigens. In addition, to quantify subpopulations of immunocompetent cells, the activity of lymphocytes was measured. Lymphocytes were stimulated by mitogen phytohemagglutinin and their proliferation measured by a flow cytometric method. No statistically significant differences between the control and exposed persons were found. In both study groups all immune parameters were within normal ranges.

Effects of sublethal radiation on bone marrow cells: induction of apoptosis and inhibition of antibody formation.

As part of the study to investigate the mechanism of radiation-induced immunosuppression, the survival and functional ability of bone marrow cells was analyzed by exposing C57B1/6 mice whole body to 2.0-Gy gamma-rays. There was a rapid induction of DNA fragmentation in the total bone marrow cells and the kinetics indicated that apoptosis reached a peak by 4 h and then dropped back to normal control levels within 10 h after irradiation. To determine the functional ability of bone marrow cells which survive the radiation treatment, animals were immunized with antigen trinitrophenyl (TNP)-lipopolysaccharide. There was a significant decrease of anti-TNP plaque-forming cells in the bone marrow of irradiated mice compared to control animals. Flow cytometric analysis of bone marrow revealed a significant depletion of both immature (B220+, Ig-) as well as mature (B220+, Ig+) B cells compared to control group. In summary, the present study showed that sublethal whole body irradiation inhibits antibody responses elicited by bone marrow cells. This decreased immune response may have been due to depletion of B lineage subsets as well as generalized apoptosis in the entire bone marrow cells.

Radioiodine-induced changes in lymphocyte subsets in patients with differentiated thyroid carcinoma.

This study evaluated changes in lymphocyte subsets in patients with thyroid carcinoma who received iodine-131 for diagnostic and therapeutic purposes. Twenty thyroid cancer patients were entered in the study after total thyroidectomy: ten patients (group A) underwent whole-body scintigraphy with 185 MBq of (131)I and the other ten (group B) received 3700 MBq of (131)I therapy. All patients were in a hypothyroid state at the time of administration of (131)I and started L-thyroxine 150 microg/day 3 days after (131)I administration. Free and bound triiodothyronine and thyroxine, thyroid-stimulating hormone, thyroglobulin, thyroglobulin antibodies, thyroid peroxidase/microsomal antibodies, white blood cell, lymphocyte counts and lymphocyte subsets were serially determined at baseline and at days 2, 7, 15, 30 and 60 after (131)I administration. Twenty healthy age- and sex-matched individuals were used as a reference population for lymphocyte subset values. In group A only a reduction in NK cells at days 7 (P=0.043) and 15 (P=0.037) was observed. In group B, patients showed a delayed reduction in the total lymphocyte count at days 15, 30 and 60 (P=0.008, 0.004 and 0. 018, respectively), and a decrease in B cells throughout the study (at days 7, 15, 30 and 60: P=0.006, 0.0017, 0.0017 and 0.0017 respectively). A transient decrease in NK cells was observed at days 15 (P=0.025) and 30 (P=0.008). Among T cells, the helper phenotype (CD4+) was mainly affected, resulting in a reduction in the CD4+/CD8+ ratio at day 60 (P=0.046). Comparing the two groups, the numbers of B lymphocytes at day 30 (P=0.023) and NK cells at days 2 (P=0.037) and 30 (P=0.023) were significantly lower in group B. Neither group showed any clinical sign of immunosuppression during the follow-up period. In patients with thyroid cancer the sensitivity of lymphocytes to the effects of (131)I administered for diagnostic or therapeutic purposes depends upon lymphocyte phenotype and (131)I activity. NK cells are the most radiosensitive cells, being reduced even by low (131)I activity. At higher activity all subtypes show a reduction, which is more marked and prolonged for B lymphocytes and, to a lesser extent, for T-helper lymphocytes. These changes do not result in clinically relevant immunosuppression.

Ultraviolet radiation-induced suppression of natural killer cell activity is enhanced in xeroderma pigmentosum group A (XPA) model mice.

Xeroderma pigmentosum group A gene-deficient mice easily develop skin cancers by ultraviolet radiation. Natural killer cells play an important part in tumor surveillance. To study whether ultraviolet radiation-induced suppression of natural killer cell function is involved in the high incidence of skin tumors in patients with xeroderma pigmentosum, we analyzed the number and activity of natural killer cells in ultraviolet B-irradiated xeroderma pigmentosum A model mice. The number of natural killer cells in peripheral blood significantly decreased after ultraviolet B-irradiation only in xeroderma pigmentosum A mice, but those in the spleen were not affected. As compared with the wild-type mice, the xeroderma pigmentosum A mice displayed a higher level of spontaneous splenic natural killer cell activity (10%-15% vs 3%) and inducible natural killer activity (30%-50% vs 20%-25%) after injection of polyinosinic:polycytidylic acid. At 24 h after the last irradiation of three and five daily consecutive exposures to 500 mJ per cm2-ultraviolet B, however, the natural killer activity in xeroderma pigmentosum A mice decreased to 60 and 30% of the preirradiated level, respectively, but it did not in the wild-type mice. The depression of natural killer activity in xeroderma pigmentosum A mice recovered to a normal level at 10 and 15 d after the last irradiation, respectively. The high incidence of skin cancers in xeroderma pigmentosum patients may be mainly due to a defect in the repair of ultraviolet-damaged DNA of cutaneous cells, and possibly also due to an intensified ultraviolet-induced immunosuppression. Moreover, the present study suggests that the enhanced ultraviolet-induced impairment of natural killer function could be partially involved in cancer development.

Impact of localized radiotherapy on blood immune cells counts and function in humans.

Immune cells subsets were prospectively analyzed after localized radiotherapy (LRT). LRT reduced the levels of all lymphocyte subsets, with B-cells and naive T-cells being most sensitive. Lymphocyte function was suppressed, but still within the normal range. Rapid recovery of cytotoxic T-cells/natural killer cells after LRT and the functional suppression within normal levels explains the low incidence of infections after LRT.

Cellular immune defect caused by postsurgical radiation therapy in patients with head and neck cancer.

The effects of locoregional postoperative radiation therapy (60 Gy on average) on cellular immunity were investigated in 11 patients with squamocellular carcinomas of the oral cavity, pharynx, or larynx. During radiation treatment, the total lymphocyte counts, CD8+ T-lymphocyte count, and especially CD4+ T-lymphocyte count decreased significantly. The mean CD4+ T-lymphocyte counts dropped from an average of 739/microl to 183/microl (p <0.001), and the CD4+/CD8+ quotient also decreased significantly. Not only the lymphocyte counts but also the in vitro lymphocyte stimulation responses to several mitogens decreased, with reductions averaging 10% to 50% of normal responses by the end of radiation therapy. Within 3 to 4 weeks after radiation therapy, the CD4+ T-lymphocyte counts and the in vitro lymphocyte stimulation responses showed a tendency toward normalization. This study shows that postoperative locoregional radiation therapy in patients with head and neck cancer induces a severe generalized impairment of cellular immunity.

[Effect of mild-frequency ultraviolet radiation on mice splenic lymphocytes]. / Deistvie srednevolnovogo ul'trafioletovogo izlucheniia na limfotsity selezenki myshei.

Investigation of the action of mid-frequency ultraviolet radiation (302 nm) on isolated lymphocytes of mice spleen pointed out that intracellular pH of lymphocytes starts decreasing from 0.3 J/cm2. Dose rise above 2.5 J/cm2 increases the number of cells with damaged plasmatic membrane within the lymphocyte population and aggravates the ability of cells to accumulate fluorescein fluorochrome as a product of intracellular fluorescein diacetate hydrolysis.

Elevated expression of Bcl-2 and Bcl-x by intestinal intraepithelial lymphocytes: resistance to apoptosis by glucocorticoids and irradiation.

Administration of glucocorticoids or exposure to ionizing radiation in vivo results in a rapid cell death of thymocytes. We report that murine small intestinal intraepithelial lymphocytes (IEL) are resistant to both steroid- and radiation-induced deletion. This is due to resistance to apoptosis, as evidenced by the absence of detectable apoptotic IEL nuclei in situ after in vivo glucocorticoid treatment. IEL express normal levels of glucocorticoid receptors and these receptors bind [3H]dexamethasone to equivalent levels as other lymphocyte populations. Thus, their survival is due to post-receptor signaling mechanisms. Many IEL express high levels of Bcl-2 and that of these Bcl-2high IEL are largely TCR gamma delta +. Those IEL that do express high levels of Bcl-2 are CD8 alpha + beta - CD4-. In addition, IEL express Bcl-x, another protein shown to be involved in the protection of cells from apoptotic signals. IEL represent the first lymphocyte population in vivo shown to have high levels of expression of both molecules, that otherwise occur only in activated lymphocytes in vitro. These data suggest that the Bcl-2+Bcl-x+ IEL are activated cells and not an effete population of cells necessarily destined to die. Also, the high levels of Bcl-2 and Bcl-x in this in vivo activated population supports the in vitro correlate of protection from activation-induced cell death.

Altered erythrocytes and a leaky block in B-cell development in CD24/HSA-deficient mice.

The heat stable antigen (HSA, or murine CD24) is a glycosyl phosphatidylinositol-linked surface glycoprotein expressed on immature cells of most, if not all, major hematopoietic lineages, as well as in developing neural and epithelial cells. It has been widely used to stage the maturation of B and T lymphocytes because it is strongly induced and then repressed again during their maturation. Terminally differentiated lymphocytes, as well as most myeloid lineages, are negative for HSA. Erythrocytes are an exception in that they maintain high levels of HSA expression. HSA on naive B cells has been shown to mediate cell-cell adhesion, while HSA on antigen-presenting cells has been shown to mediate a costimulatory signal important for activating T lymphocytes during an immune response. Here, we characterize mice that lack a functional HSA gene, constructed by homologous recombination in embryonic stem cells. While T-cell and myeloid development appears normal, these mice show a leaky block in B-cell development with a reduction in late pre-B and immature B-cell populations in the bone marrow. Nevertheless, peripheral B-cell numbers are normal and no impairment of immune function could be detected in these mice in a variety of immunization and infection models. We also observed that erythrocytes are altered in HSA-deficient mice. They show a higher, tendency to aggregate and are more susceptible to hypotonic lysis in vitro. In vivo, the mean half-life of HSA-deficient erythrocytes was reduced. When infected with the malarial parasite Plasmodium chabaudi chabaudi, the levels of parasite-bearing erythrocytes in HSA-deficient mice were also significantly elevated, but the mice were able to clear the infection with kinetics similar to wild-type mice and were immune to a second challenge. Thus, apart from alterations in erythrocytes and a mild block in B-cell development, the regulated expression of HSA appears to be dispensable for the maturation and functioning of those cell lineages that normally express it.

Effects of low power laser-irradiation on differential blood count and body temperature in endotoxin-preimmunized rabbits.

Low power laser irradiation has been shown to have various immune-modulatory effects under in vitro conditions but little is known about such effects in animal models. Escherichia coli endotoxin-preimmunized rabbits were used to determine the influence of transcutaneously applied low power laser light on differential blood count and rectal temperature. After three initial immunizations animals were either boostered with 5 ng/kg of endotoxin or injected with pyrogen-free saline and subsequently underwent irradiation using two different wavelengths of red laser light and sham irradiation, respectively. Differential blood count of laser-treated animals was characterized by significantly higher lymphocyte values and lower neutrophil values at twenty hours (boostered rabbits) and twenty-three hours (non-boostered rabbits) after irradiation. Differential blood cell counts returned to baseline values within 23 hours in the boostered animals, whereas in the non-boostered rabbits lymphocytes showed a trend to further increase. Recording of rectal temperature revealed a further rise after laser application, changes being of greater magnitude and longer duration in the non-boostered animals. These results seem to indicate that a single low power laser irradiation can modulate immune-responses depending on the immunological status of the organism.

Immune function and host defense in rodents exposed to 60-Hz magnetic fields.

This study was conducted to evaluate the influence of subchronic exposure to pure, linearly polarized 60-Hz magnetic fields (MF) on the host immune response in mice. The experimental design was as follows: three groups were exposed continuously (18.5 hr/day) to MF at field strengths of 0.02, 2, or 10 gauss (G), one group was exposed intermittently (1 hr on/1 hr off) to MF at a field strength of 10 G, and one group served as a sham control. Experimental endpoints included spleen and thymus weights and cellularity, antibody-forming cell (AFC) response, delayed-type hypersensitivity (DTH) response, splenic lymphocyte subset analysis, susceptibility to infection with Listeria monocytogenes, and natural killer (NK) cell activity. No differences in body weight, lymphoid organ weight, or lymphoid organ cellularity were observed in any MF-exposed group in comparison to sham controls. Likewise, no statistically significant differences were found in comparisons of AFC responses. Isolated statistically significant differences from control were observed in MF-exposed mice in the DTH assay, although no clear dose-related pattern of altered activity was seen. Splenic lymphocyte subset parameters examined were within normal limits in all groups, and no differences between control and MF-exposed mice were found. Host resistance to bacterial infection was not altered at any MF exposure examined in this study. Finally, although apparently dose-related, statistically significant alterations were observed in an initial study of NK cell function, repeat studies failed to demonstrate a consistent pattern of alteration.

Susceptibility to clinically manifest cyclosporine A (CsA)-induced autoimmune disease is associated with interferon-gamma (IFN-gamma)-producing CD45RC+RT6- T helper cells.

Lethally irradiated Lewis (LEW) rats reconstituted with syngeneic bone marrow and given CsA for a 4-week period, develop, upon withdrawal of CsA, a graft-versus-host-like disease, so-called CsA-induced autoimmunity (CsA-AI). This T cell-mediated autoimmune disease is thymus-dependent; it is generally held that this disease is a consequence of aberrant T cell recovery brought about by CsA. In this study we determined mononuclear cell subsets phenotypically by tri-colour flow cytometry. A strong decrease in recent thymic emigrants (Thy1.1+, TCR alpha beta +) was observed as a consequence of CsA treatment, eventually resulting in decreased absolute peripheral T cell numbers. In these rats no altered CD4:CD8 T cell ratio was observed before onset of CsA-AI; CD4+ and CD8+ cells consisted predominantly of monocytes (CD4dim+, TCR alpha beta-) and natural killer cells (CD8+, TCR alpha beta-), respectively. LEW rats, x-irradiated, syngeneic bone marrow-reconstituted and treated with CsA, showed a marked and persistent, relative expansion of mature CD45RC+, RT6- Th cells. In contrast, Brown-Norway rats treated in a similar fashion, or LEW rats subjected to either CsA treatment or x-irradiation, did not show a comparable expansion of mature CD45RC+, RT6- Th cells, nor did these animals develop CsA-AI. The CD45RC+, RT6- Th cells produced IL-2, and moreover constituted the only Th subset producing IFN-gamma upon stimulation, and therefore were considered as Th1-like effector cells. These results are consistent with the view that a persistent preponderance of Th1 cells and not the mere presence of autoreactive cells determines whether or not clinically manifest CsA-AI will occur.

[A comparative evaluation of the content of T-lymphocyte subpopulations, alpha 1-thymosin and autoantibodies to epithelial thymic cells in the personnel in the 30-kilometer control zone of the accident at the Chernobyl Atomic Electric Power Station]. / Sravnitel'naia otsenka soderzhaniia subpopuliatsii T-limfotsitov, al'fa 1-timozina i autoantitel k épitelial'nym kletkam timusa u personala 30-kilometrovoi zony kontrolia avarii na Chernobyl'skoi AES.

A significant decrease in mean number of CD5+, CD8(+)-lymphocytes in persons, who worked in 30-km zone of Chernobyl nuclear power station was revealed. A significant increase in percent of CD5+, CD4(+)-cell percents was observed in workers, who worked for 1,2-2,5 years in zone, but absolute number, were decreased comparing a control and data received in people, who have just arrived to work in 30-km zone. The positive correlation exists between the percent of lymphocytes and years of service in 30-km zone. The lower level of alpha 1-thymosine was revealed in serum of the persons, who worked in zone for 4.5-5 years than data received in people, who worked for 0.5 year. Increase level of serum autoantibodies reacting with thymic epithelial cell was detected in men, who worked in zone for 3-3.5 years. In persons, who worked more 5 years and have just arrived in zone identical data were received.

Severe lymphocytopenia and interstitial pneumonia in patients treated with paclitaxel and simultaneous radiotherapy for non-small-cell lung cancer.

PURPOSE: In a phase II trial with paclitaxel and simultaneous radiotherapy in non-small-cell lung cancer (NSCLC) patients, an unexpected high incidence of interstitial pneumonias was observed. The type of immunodeficiency associated with this treatment approach is characterized. PATIENTS AND METHODS: Fifteen patients with inoperable stage IIIA/B NSCLC were treated with paclitaxel as a 3-hour infusion on day 1 in weeks 1 to 3 and 6 to 8 at dose levels between 50 mg/m2 and 86 mg/m2 and with simultaneous radiotherapy in daily doses of 2 Gy, 5 days per week, in weeks 1 to 3 and 6 to 8 up to a total dose of 56 Gy. Hematologic parameters and lymphocyte subsets were monitored. RESULTS: Fourteen patients are assessable for response. The overall response rate was 78%, with four major responses, six partial remissions, and four minor responses. The major toxic effect observed was a moderate to severe protracted lymphocytopenia (380 +/- 310/microL) in all patients. Seven patients developed moderate to severe interstitial pneumonia; one had an additional herpes zoster infection, while an eighth patient had a cytomegalovirus infection. During treatment, all lymphocyte subsets were reduced, as follows (n = 9, mean +/- SD): CD4+ T cells (100 +/- 90/microL), CD8+ T cells (130 +/- 160/microL), natural killer (NK) cells (70 +/- 80/microL), and B cells (20 +/- 10/microL). Thus, the most pronounced toxicity was seen in CD4+ T and B cells. There was no recovery of lymphocyte subsets during a 3-month follow-up period. CONCLUSION: Paclitaxel with simultaneous radiation induces lymphocytopenia and promotes opportunistic infections. Long-term antibiotic and antimycotic prophylaxis is recommended. Whether the lymphocytopenia is an additive effect of paclitaxel and radiation or whether it can be induced by low-dose weekly paclitaxel alone remains to be determined.

The effect of UV-B irradiation on secondary epidermal infection of mice with herpes simplex virus type 1.

Previous studies have indicated that suberythemal ultraviolet B (UV-B) irradiation of C3H mice before primary infection with herpes simplex virus (HSV) type 1 does not result in increased morbidity or mortality, but a suppressed delayed type hypersensitivity (DH) to the virus can be demonstrated. Any effect of UV radiation on pathogenesis during secondary epidermal HSV infection has not been previously examined. Mice were immunized by subcutaneous injection of inactivated HSV and, 5 days later, one group was UV-B-irradiated. The next day all mice were challenged epidermally with HSV. Most of the mice (92%) in the irradiated group developed severe lesions, whilst 59% of the non-irradiated group had mild lesions and 30% no lesions. Infectious virus was not isolated from the adrenal glands after challenge in either group. In addition, the DH to the virus was not affected by the UV exposure. The numbers of lymphocytes and dendritic cells in the lymph nodes draining the site of epidermal infection were increased in the UV group compared with the non-irradiated group. Following challenge, the percentage of CD4+ and CD8+ lymphocytes in lymph nodes was unaltered but the MHC class II expression on dendritic cells in these lymph nodes was reduced by UV exposure. The lymphoproliferative response in vitro of lymph node cells revealed a suppressed response to HSV and to the mitogen concanavalin A in the irradiated group. Thus, UV irradiation prior to epidermal secondary infection with HSV led to more severe infections due, perhaps, to a modulation in local antigen presentation.

Analysis of alloreactive helper T lymphocyte precursor frequencies. Influence of interleukin-2 produced by the stimulating cells.

In the determination of alloreactive helper T lymphocyte precursor frequencies (HTLpf) by limiting dilution analysis (LDA), peripheral blood mononuclear cells (PBMC) are used as stimulating cells. Interleukin-2 (IL-2) production by stimulating T cells constitute a source of error in these assays. We found that 100-150 Gy of gamma irradiation was required to abrogate IL-2 production by stimulating PBMC. This dose of irradiation, however, greatly reduced their allostimulatory capability. Here we describe how non-irradiated PBMC, immunomagnetically depleted of T cells and thus of IL-2 producing cells, can be used as stimulators in assays to determine alloresponsive HTLpf.

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.