Marrow toxicity of fractionated vs. single dose total body irradiation is identical in a canine model

PURPOSE

We explored in dogs the marrow toxicity of single dose total body irradiation delivered from two opposing 60Co sources at a rate of 10 cGy/min and compared results to those seen with total body irradiation administered in 100 cGy fractions with minimum interfraction intervals of 6 hr. Dogs were not given marrow transplants.

RESULTS

We found that 200 cGy single dose total body irradiation was sublethal, with 12 of 13 dogs showing hematopoietic recovery and survival. Seven of 21 dogs given 300 cGy single dose total body irradiation survived compared to 6 of 10 dogs given 300 cGy fractionated total body irradiation (p = .18). One of 28 dogs given 400 cGy single dose total body irradiation survived compared to none of six given fractionated radiation (p > .20). With granulocyte colony stimulating factor administered from day 0-21 after 400 cGy total body irradiation, most dogs survived with hematological recovery. Because of the almost uniform success with granulocyte colony stimulating factor after 400 cGy single dose total body irradiation, a study of granulocyte colony stimulating factor after 400 cGy fractionated total body irradiation was deemed not to be informative and, thus, not carried out. Additional comparisons between single dose and fractionated total body irradiation were carried out with granulocyte colony stimulating factor administered after 500 and 600 cGy of total body irradiation. As with lower doses of total body irradiation, no significant survival differences were seen between the two modes of total body irradiation, and only 3 of 26 dogs studied survived with complete hematological recovery. Overall, therefore, survival among dogs given single dose total body irradiation was not different from that of dogs given fractionated total body irradiation (p = .67). Similarly, the slopes of the postirradiation declines of granulocyte and platelet counts and the rates of their recovery in surviving dogs given equal total doses of single versus fractionated total body irradiation were indistinguishable.

CONCLUSION

Within the limitations of the experimental design, we conclude that single-dose and fractionated total body irradiation have comparable marrow toxicity in dogs.

Risk factors for the development of secondary malignancies after marrow transplantation

Bone marrow transplantation is associated with several risk factors that may predispose patients to the development of a new malignancy post-transplant. Several types of malignancies, including lymphoproliferative disorders, new leukemias, and solid tumors have been observed. Risk factors include treatment of acute graft-versus-host disease with monoclonal or polyclonal antibodies, total body irradiation, HLA nonidentity, and T-cell depletion. It is currently unknown whether longer follow-up will show a further increase or decline in the incidence of secondary tumors.

Inhibition of canine NK activity by anti-CD18 monoclonal antibody, UV irradiation and cyclosporine

Previous studies from our laboratory have shown that large granular lymphocytes (LGL) mediate natural killer cell (NK) activity in dogs and that host LGL are associated with graft failure in a canine model of histoincompatible marrow transplantation. We now report studies on the effects of anti-CD18 monoclonal antibody (MAB) 60.3, ultraviolet (UV) irradiation and cyclosporine (CSP) on in vitro canine NK function. Treatment with the murine anti-CD18 MAB 60.3 produced marked diminution in lytic function (mean 79% inhibition). UV irradiation at doses greater than 1.6 millijoules/cm2 completely eliminated NK function; this effect was not due to defective effector/target conjugate formation. CSP also inhibited NK function, although in vitro lytic activity was not completely blocked (mean of 44% inhibition) at concentrations which correspond to those achieved in vivo (400 ng/mL). These observations form a basis for future investigations aimed at preventing graft failure in the canine model of histoincompatible marrow transplantation by blocking NK function in vivo.

Methotrexate, cyclosporine, or both to prevent graft-versus-host disease after HLA-identical sibling bone marrow transplants for early leukemia?

Optimal prophylaxis of graft-versus-host disease (GVHD) is controversial. We compared efficacy of three posttransplant immune suppressive regimens in 2,286 recipients of HLA-identical sibling bone marrow transplants for acute lymphoblastic leukemia (ALL) in first remission, acute myelogenous leukemia (AML) in first remission, or chronic myelogenous leukemia (CML) in first chronic phase. Six hundred forty received methotrexate, 977 received cyclosporine, and 669 received combined cyclosporine and methotrexate. In children, the three regimens resulted in similar outcomes. In adults, cyclosporine and methotrexate had comparable risks of acute and chronic GVHD. Compared with methotrexate, cyclosporine was associated with less interstitial pneumonia (relative risk [RR] = 0.6; P < .001), less treatment-related mortality (RR = 0.6; P < .001), more relapses (RR = 1.6; P < .05), and less treatment failure (relapse or death from any cause; RR = 0.7; P < .001). Different effects were observed in different leukemias. In ALL, the rate of leukemia relapse was increased with cyclosporine versus methotrexate, with no effect on other outcomes. In AML and CML, interstitial pneumonia, treatment-related mortality, and treatment failures were decreased with cyclosporine, with no increase in relapse. Similar analyses comparing cyclosporine plus methotrexate with cyclosporine alone showed that adults receiving the combination had less acute GVHD (RR = 0.5; P < .001), less chronic GVHD (RR = 0.7; P < .01), and less interstitial pneumonia (RR = 0.7; P < .001). Treatment failure (RR = 0.8; P < .05) was marginally reduced. Separate analyses in ALL and AML showed less acute GVHD with combined therapy, but no significant effect on other outcomes. In CML, acute GVHD, interstitial pneumonia, treatment-related mortality, and treatment failure were decreased with combined therapy.

Effects of recombinant canine stem cell factor, a c-kit ligand, and recombinant granulocyte colony-stimulating factor on hematopoietic recovery after otherwise lethal total body irradiation

The effects of recombinant canine stem cell factor (rcSCF) on hematopoiesis were studied in normal dogs and in dogs given otherwise lethal total body irradiation (TBI) without marrow transplant. Results were compared with previous and concurrent data with recombinant granulocyte colony-stimulating factor (rG-CSF). Four normal dogs received 200 micrograms rcSCF per kilogram body weight daily either by continuous intravenous infusion for 28 days (n = 2) or by subcutaneous (SC) injection in two divided doses for 20 days (n = 2). All dogs showed at least a twofold increase in peripheral blood neutrophil counts starting approximately 7 days after the initiation of treatment. Hematocrit level and monocyte, lymphocyte, eosinophil, reticulocyte, and platelet counts were not elevated. Marrow sections after rcSCF treatment showed panhyperplasia. The only toxicity was facial edema during the first few days of rcSCF administration, presumably caused by mast cell stimulation. Ten dogs were given 400 cGy TBI at 10 cGy/min from two opposing 60Co sources. They were given no marrow infusion and received 200 micrograms/kg/d rcSCF SC in two divided doses for 21 days starting within 2 hours of TBI. Five of the 10 dogs showed complete and sustained hematopoietic recovery and survived as compared with 1 of 28 control dogs not receiving growth factor (P < .005). RcSCF treatment allowed for hematopoietic recovery in two of seven dogs administered 500 cGy of TBI but in none of five dogs given 600 cGy of TBI. Results with rcSCF are similar to those obtained with rG-CSF. The rate of neutrophil recovery in rcSCF-treated dogs after 400 cGy TBI was not different from that of rG-CSF-treated dogs (P = .65), but the rate of platelet recovery was faster (P = .06) in the rcSCF-treated animals. Combined treatment with rcSCF and rcG-CSF after 500 cGy TBI did not result in strongly improved survival as compared with results obtained with either factor alone.

Pharmacologic, toxicologic, and marrow transplantation studies in dogs given succinyl acetone

A novel immunosuppressant, succinyl acetone (4,6-dioxoheptanoic acid), was studied in dogs. Results with bolus intravenous injections at doses ranging from 50 to 1600 mg/kg showed dose-dependent alpha and beta half-lives, ranging from 30 to 80 min and 7 to 27 hr, respectively. Results suggested that continuous i.v. infusion was necessary to maintain constant plasma levels. Four dogs were given 9.2 Gy total-body irradiation and autologous marrow transplants along with continuous i.v. infusion of succinyl acetone at 50, 100, 200, or 400 mg/kg/day for 21 days, and all four had rapid, sustained hematopoietic engraftment. However, two of the four dogs receiving 200 and 400 mg succinyl acetone/kg/day, respectively, developed bilateral hind-limb ataxia, with histologically confirmed cerebellar lesions in the dog given the higher dose, thus establishing a potential dose-limiting neurotoxicity. Prevention of graft-versus-host disease was studied in recipients of allogeneic marrow. Dogs were given 9.2 Gy TBI, followed by hematopoietic grafts from unrelated DLA-nonidentical or DLA-haploidentical littermate dogs. Succinyl acetone was given as continuous infusion for 21 days after transplant at doses of 100-300 mg/kg/day. Starting succinyl acetone on the day of marrow infusion in four dogs failed to prevent rapid onset of acute GVHD, and dogs survived no longer than controls. Starting succinyl acetone 3 days before transplant delayed the onset of acute GVHD and prolonged survival significantly compared with that of dogs not given postgrafting immunosuppression (P = 0.008); survival was comparable to that in previously reported dogs given either methotrexate or cyclosporine as postgrafting immunosuppression (P = 0.88 and 0.99, respectively). Seven of the sixteen allogeneic recipients developed evidence of neurotoxicity during succinyl-acetone infusion. Neurological dysfunctions were manifested by hind-limb ataxia and posterior paresis. In conclusion, succinyl acetone significantly delayed the onset of GVHD and prolonged survival of DLA-nonidentical marrow graft recipients but did not induce graft-host tolerance and was associated with dose-limiting neurotoxicity.

Acute graft-versus-host disease: analysis of risk factors after allogeneic marrow transplantation and prophylaxis with cyclosporine and methotrexate

Previous studies of risk factors for acute graft-versus-host disease (GVHD) involved patients receiving predominantly single-agent prophylaxis. Therefore, a retrospective analysis was performed on 446 patients, from a single institution, who received transplants of marrow from HLA-identical siblings and the combination of cyclosporine (CSP) and methotrexate (MTX) to determine risk factors for acute GVHD associated with this more effective form of GVHD prophylaxis. The incidences of Grades II-IV and Grades III-IV (severe) acute GVHD were 35% and 16%, respectively. Increased clinical grades of acute GVHD in patients without advanced malignant disease were associated with a decreased survival. In a multivariate Cox regression analysis, risk factors associated with the onset of Grades II-IV acute GVHD were sex mismatch and donor parity (P = .001), increased dose of total body irradiation (TBI) (P = .001), and reduction to less than 80% of the scheduled dose of MTX (P = .02) or CSP (P = .02). The multivariate analysis indicated a relative risk of 1.37 for acute GVHD in a group defined as having advanced malignant disease at transplant; however, this difference failed to reach conventional levels of statistical significance (P = .07). Reduction of MTX and CSP occurred in up to 36% and 44% of patients, respectively, primarily because of renal or hepatic dysfunction. The periods of increased risk for the onset of acute GVHD were up to 1 week after a reduction of MTX and 2 weeks after a reduction in CSP. When only patients who developed Grades II-IV acute GVHD were considered, the more severe acute GVHD of Grades III-IV was associated with increased patient age of 40 years or greater (P = .05) and dose reductions of CSP (P = .008). Serologic status of patient and donor for cytomegalovirus (CMV), HLA antigens in the A and B loci, and isolation in a laminar air flow room during marrow transplantation, all previously identified as risk factors for acute GVHD, were not confirmed as risk factors in this study population. The toxicity of MTX and CSP and the development of acute GVHD from inadequate immunosuppression because of dose reduction warrants further trials with potentially less toxic immunosuppressive agents. Risk factors for acute GVHD should be considered in clinical management and in the design of clinical trials.

Specific marrow ablation before marrow transplantation using an aminophosphonic acid conjugate 166Ho-EDTMP

166Holmium ethylenediaminetetramethylene phosphonic acid (166Ho-EDTMP) is a short-lived beta-emitting radionuclide complexed to an aminophosphonate ligand that we have investigated in a canine model as a potential agent for specific marrow ablation before marrow transplantation. After intravenous injections, 166Ho-EDTMP distributed principally to bone and after 24 hours the concentrations of 166Ho-EDTMP in bone were more than 200-fold higher than in any other organ. Increasing dosages of 166Ho-EDTMP led to increasingly prolonged and severe myelosuppression, but myeloablation was not achieved. Histologic examination of recovering animals suggested that the spleen may have acted as a reservoir for circulatory hematopoietic precursors. Four splenectomized animals administered 20 to 30 mCi/kg 166Ho-EDTMP without marrow transplantation died with marrow aplasia, while four splenectomized animals administered similar dosages of 166Ho-EDTMP followed by autologous transplantation recovered. The dose-limiting toxicity of 166Ho-EDTMP appeared to be marrow stromal damage resulting in myelofibrosis, which was reversible. These results suggest that 166Ho-EDTMP can be used to specifically ablate marrow function before marrow transplantation.

Etoposide, cyclophosphamide and fractionated total body irradiation as a preparative regimen for marrow transplantation in patients with advanced hematological malignancies: a phase I study

Thirty-seven patients with advanced hematologic malignancy were entered into a phase I study designed to define a maximum tolerable dose (MTD) of etoposide (VP-16) and cyclophosphamide (CY) combined with 12 Gy fractionated total body irradiation (TBI) as preparation for marrow transplantation from an HLA-identical sibling (n = 13) or with cryopreserved autologous marrow (n = 24). Dose levels ranged from 36 mg/kg of VP-16 combined with 67 mg/kg of CY to 52 mg/kg of VP-16 combined with 103 mg/kg of CY followed by 12 Gy TBI. The MTD for allogeneic marrow recipients was 36 mg/kg of VP-16 + 52 mg/kg of CY followed by 12 Gy TBI and for autologous marrow recipients 44 mg/kg of VP-16 + 103 mg/kg CY followed by 12 Gy TBI. Pulmonary and liver toxicities were dose limiting. All of 31 evaluable patients transplanted in relapse achieved a complete remission. However, in all but three of these patients the disease relapsed 28-899 (median 110) days post-transplant. Currently, six of 24 autologous marrow recipients are surviving, three in remission 256, 340 and 764 days post-transplant. None of 12 allogeneic marrow recipients have survived. In conclusion, a preparative regimen combining 44 mg/kg of VP-16 + 103 mg/kg CY followed by 12 Gy TBI is well tolerated by autologous marrow recipients and 36 mg/kg VP-16 + 67 mg/kg CY followed by 12 Gy TBI is well tolerated by allogeneic marrow recipients.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of post-methotrexate folinic acid rescue on regimen-related toxicity and graft-versus-host disease after allogeneic bone marrow transplantation

Thirty-two patients undergoing related-donor bone marrow transplantation (BMT) received cyclosporine (CSP) and methotrexate (MTX) with folinic acid rescue (FAR) as graft-versus-host disease (GVHD) prophylaxis. Fifty consecutive related-donor BMT patients given the CSP/MTX combination without FAR were utilized as historical controls. Patients receiving FAR experienced earlier engraftment, with absolute neutrophil count greater than 0.5 x 10(9)/l at a median of 17 days (vs 21 days in controls, p = 0.002). The day of last platelet transfusion was earlier in the FAR group (median of 14 days vs 17 days in controls, p = 0.01). Compared with the control group, patients receiving FAR had a lower incidence of grade II-IV stomatic (53% vs 78%, p = 0.04) and hepatic (25% vs 56%, p = 0.01) regimen-related toxicity. In the FAR group, 70% required total parenteral nutrition vs 92% of controls (p = 0.02). Broad-spectrum antibiotics were given to FAR patients for a median of 21 days (vs 23 days in controls, p = 0.09). The incidence of grade II-IV acute GVHD was similar in the FAR and control populations (45% and 35%, respectively, p = NS) as was the incidence of chronic GVHD (62% vs 55%, respectively, p = NS). Estimated event-free survival is 59% for FAR patients (median follow-up 64 weeks) and 58% for controls (median follow-up 109 weeks, p = NS). FAR reduces regimen-related toxicity in patients receiving CSP/MTX acute GVHD prophylaxis without significantly influencing GVHD incidence or event-free survival.

Diagnostic use of molecular probes before and after marrow transplantation

The application of molecular techniques to disease diagnosis and histocompatibility testing before marrow transplantation and to the evaluation of engraftment and immune reconstitution, and to the identification of infectious and neoplastic complications following transplantation has revolutionized the approach to the care of marrow transplant patients. The techniques of restriction fragment length polymorphism analysis using informative restriction enzymes, the Southern blot technique, and of polymerase chain reaction-amplified DNA and oligonucleotide probe hybridization methods now enable the study of donor and host cells at the nucleotide level.

Marrow transplantation for Fanconi anemia with or without leukemic transformation: an update of the Seattle experience

Between March 1973 and August 1990, 17 patients with Fanconi anemia (FA) underwent bone marrow transplantation in Seattle. Marrow donors were HLA identical siblings (n = 14), phenotypically HLA identical parents (n = 2) and a one antigen mismatched parent (n = 1). Patients with no evidence of leukemic transformation (n = 12) were conditioned with 140-200 mg/kg cyclophosphamide (CY). Of five patients with leukemic transformation, four received CY (120 mg/kg) plus 12 Gy fractionated total body irradiation and one patient received busulfan (14 mg/kg) and CY (100 mg/kg). All patients engrafted; however, one patient whose sibling donor's cells showed variable results when assayed for chromosome instability required two additional marrow infusions. Toxicity associated with the conditioning regimen included severe oral mucositis (n = 14), hemorrhagic cystitis (n = 11) and diffuse erythroderma (n = 3). Seven of the 12 patients without leukemic transformation are surviving 1-17 years (median = 5 years) after transplant, with an estimated survival probability at 5 years of 65% (95% CI 0.31; 0.85). Two patients developed squamous cell carcinoma of the tongue greater than 10 years post-transplant. One of these patients died at 10.3 years as a result of the malignant process, and the other is disease free more than 12 years post-transplant. Of the five patients with leukemic transformation, one is alive at 8 years. These data demonstrate that marrow transplantation can offer long-term survival for patients with FA, engraftment can be achieved with reduced doses of CY in FA patients, and less toxic preparative regimens are needed for FA patients who have developed leukemic transformation.

Marrow transplantation following escalating doses of fractionated total body irradiation and cyclophosphamide--a phase I trial

Thirty-six patients with advanced hematologic malignancy were entered into a Phase I study designed to define the maximum tolerated dose of unshielded total body irradiation delivered from dual 60 Cobalt sources at an exposure rate of 8 cGy/min and given in fractions twice daily for total doses ranging from 12 Gy to 17 Gy. All patients received cyclophosphamide, 120 mg/kg administered over 2 days before total body irradiation. Allogeneic marrow was infused from HLA-identical siblings (n = 29) or one locus HLA incompatible family members (n = 3); three patients received cryopreserved autologous marrow and one patient received syngeneic marrow. The maximum tolerated dose of total body irradiation given as 2 Gy fractions twice a day was 16 Gy. One of eight patients receiving 12 Gy, none of four receiving 14 Gy, three of 20 receiving 16 Gy, and two of four receiving 17 Gy developed severe (Grade 3-4) regimen-related toxicity. The primary dose limiting toxicity was pneumonitis, followed by veno-occlusive disease of the liver, renal impairment, and mucositis. Five patients (14%) are alive, four disease-free 798-1522 days posttransplant. Twenty (56%) relapsed posttransplant. Further investigation of regimens containing 16 Gy of hyperfractionated total body irradiation is warranted to assess anti-tumor efficacy.

Ultraviolet B-induced DNA fragmentation (apoptosis) in activated T-lymphocytes and Jurkat cells is augmented by inhibition of RNA and protein synthesis

UVB (280-320 nm) light profoundly affects cellular immunity and immunogenicity. To further characterize the interaction of UVB light with T cells, we examined UVB-induced DNA fragmentation patterns in normal T cells and the T-cell line Jurkat. Resting or preactivated peripheral blood T cells from normal donors or Jurkat cells were exposed to various doses of UVB or gamma irradiation. Cells were sampled at 0-48 h after exposure, and DNA fragmentation was analyzed by electrophoresis in agarose gels. UVB and gamma irradiation, in a dose- and time-dependent manner, induced DNA fragmentation in Jurkat cells and in T cells preactivated with phytohemagglutinin (PHA; 10 micrograms/ml) and phorbol myristate acetate (PMA; 10 ng/ml), but not in resting T cells. The presence of RNA or protein synthesis inhibitors such as actinomycin D or cycloheximide neither inhibited nor delayed DNA fragmentation; in fact, DNA fragmentation was augmented above control values. Similarly, DNA fragmentation increased in the presence of the calcium-chelating agent ethylene-glycol-bis-tetraacetic acid (EGTA) and decreased in the presence of the calcium ionophore A23187. In the presence of ethylenediaminetetraacetic acid (EDTA), DNA fragmentation decreased. In summary, these data show that UVB-induced DNA fragmentation strongly depends upon the cell activation status, and upon the presence of divalent cations other than calcium, possibly magnesium. The data indicate furthermore that in this model, inhibition of RNA or protein synthesis can induce rather than inhibit apoptosis, suggesting that the synthesis inhibitors disrupted primarily the synthesis or action of enzymes ordinarily aimed at repairing DNA fragmentation.

Hematopoietic reconstitution and prevention of graft-versus-host disease with UVB-irradiated haploidentical murine spleen and marrow cells

We investigated in a murine model whether UVB irradiation of lymphohemopoietic cells would prevent the development of graft-versus-host disease (GVHD). Preliminary experiments showed that spleen colony (CFU-S) formation by hemopoietic cells was preserved at UVB doses that eliminated lymphocyte proliferation. In a parent into F1 model, UVB irradiation (5 to 15 mJ/cm2) of spleen cells added to normal marrow cells prevented the development of GVHD, whereas all recipients given untreated spleen cells developed GVHD. Syngeneic recipients of marrow exposed to 2.5 to 10 mJ/cm2 of UVB achieved normal hemopoietic reconstitution. Based on these observations, B6D2 F1 (H-2b x H-2d) recipients were given 1,000 cGy of total body irradiation (TBI) followed by transplantation of 5 x 10(6) parental B6 (H-2b) bone marrow cells and 10 x 10(6) B6 spleen cells, either unirradiated or exposed to UVB before infusion. All mice transplanted with cells exposed to 10 or 12.5 mJ/cm2 of UVB survived without GVHD. At 2.5 and 5.0 mJ/cm2, mice showed signs of GVHD, beginning at day 30, and 100% and 80%, respectively, eventually developed chronic GVHD. At 7.5 mJ/cm2, mice had weight loss, from which 60% recovered and survived without GVHD, while 40% died with GVHD. At 15 mJ/cm2, some recipients died from graft failure, while some survived without GVHD. All surviving mice were complete donor-type chimeras. Spleen size and cellularity and in vitro lymphocyte responses correlated inversely with the development of GVHD. Mice without GVHD showed specific tolerance to skin grafts from the second parent strain, while animals with GVHD rejected their skin grafts. Thus, in a murine model UVB irradiation of transplanted hemopoietic stem cells allows for hemopoietic reconstitution and prevents GVHD.

Late failure of autologous marrow grafts in lethally irradiated dogs given anti-class II monoclonal antibody

We established a model of canine marrow autografts after 9.2 Gy total body irradiation (TBI) to study the role of class II antigens in hematopoietic stem cell growth and differentiation. Twenty dogs were given 9.2 Gy TBI, marrow, and intravenous (IV) murine anti-class II monoclonal antibody (MoAb). Infusion of 0.6 mg/kg/d of MoAb H81.98.21, an IgG2a reactive with HLA-DR, on days 0 to 4 after TBI did not prevent initial engraftment, but dogs died with late graft failure. MoAb B1F6, an IgG2a reactive with HLA-DR + DP, had no adverse effect on engraftment, although both MoAbs detect antigens on stem cells. The critical time for the effect of MoAbs is the first 4 days after transplantation. Our findings argue against several pathogenetic mechanisms, including removal of MoAb-coated stem cells by the reticuloendothelial system (RES), canine complement-mediated cytotoxic effects on stem cells, antibody-dependent cellular cytotoxicity, and inactivation of MoAb-coated cells by dog anti-mouse antibody. To distinguish between MoAb-induced damage to microenvironment (ME)/accessory cells (AC) and late graft failure from a lack of pluripotent stem cells, three dogs were given TBI, a marrow autograft, and MoAb H81.98.21 on days 0 to 4; one, given thoracic duct cells on day 6, developed graft failure; the other two, given marrow depleted of AC by L-leucyl L-leucine o-methyl ester (Leu-Leu-OMe), had sustained grafts. Findings support the notion that originally transplanted pluripotent stem cells are no longer present on day 6 and that the ME is functional and able to support newly injected stem cells.

UV irradiation of lymphocytes triggers an increase in intracellular Ca2+ and prevents lectin-stimulated Ca2+ mobilization: evidence for UV- and nifedipine-sensitive Ca2+ channels

UV irradiation induces in vitro and in vivo immunosuppression. Because mobilization of intracellular calcium ([Ca2+]i) represents a central step in cell activation and immune response, we investigated the effect of UV irradiation on Ca2+ homeostasis. Using indo-1 and cytofluorometry, [Ca2+]i kinetics in UVC- or UVB-exposed human peripheral blood leukocytes (PBL) and Jurkat cells were determined in parallel with functional assays. Increases in [Ca2+]i were observed within 2-3 h of irradiation; these increases were UV-dose dependent and reached maxima of 240% and 180% above baseline level (130 nM) for UVB and UVC, respectively. The UV-induced [Ca2+]i rise was predominantly due to influx of extracellular calcium, and it was more pronounced in T than in non-T cells. Concurrent with [Ca2+]i shifts following UV treatment, there was a loss of ability to respond to phytohemagglutinin (PHA) or to proliferate or stimulate in mixed leukocyte culture. This loss of function appeared to be related not only to UV-induced calcium shifts, but also to effects of UV irradiation on the plasma membrane. No [Ca2+]i mobilization was induced by gamma irradiation, and gamma-irradiated cells showed a normal [Ca2+]i increase in response to PHA. UV-induced Ca2+ flux into the cells was blocked by nifedipine. These data indicate that UV and gamma irradiation have different effects on lymphocyte membranes and suggest that a disruption of Ca2+ homeostasis may be involved in UV-induced lymphocyte inhibition. The data suggest, furthermore, the presence of Ca2+ channels in lymphocyte membranes that are sensitive to UV irradiation and Ca2+ channel blockers such as nifedipine.