Bleomycin-induced pulmonary fibrosis is independent of eosinophils.

Eosinophils have been shown to increase in tissues during many fibrotic conditions and consequently have been suggested to contribute to the development of fibrosis. This study tested the hypothesis that eosinophils are essential in the development of lung fibrosis in mice in response to bleomycin (BLM). Anti-IL-5 antibody was administered intraperitoneally into mice 2 h prior to endotracheal BLM inoculation and thereafter, every other day. Lung eosinophilia was evaluated by measurement of eosinophil peroxidase activity and confirmed by eosinophil counts in histologic sections. Lung fibrosis was evaluated by hydroxyproline content and confirmed by collagen staining in histological sections. Results demonstrated that BLM induced pronounced lung eosinophilia, which was maximal 7 days after BLM treatment and remained elevated through day 14, in C57B1/6 SCID mice and CBA/J mice. In contrast, eosinophilia was a minor component in the lungs of wildtype C57B1/6 mice after BLM treatment, although lung fibrosis developed similarly in all three strains of mice. Treatment with anti-IL-5 completely abrogated eosinophilia but failed to block pulmonary fibrosis induced by BLM in all mouse strains, including C57B1/6 SCID, wildtype C57B1/6 mice, and CBA/J mice. Analysis of cytokine mRNA by RNase-protection assay in C57B1/6 SCID mice indicated that BLM treatment caused enhanced expression of the cytokines, TNF-alpha, and IL-6 at days 3, 7, and 14 post-BLM inoculation, regardless of whether eosinophils were depleted by anti-IL-5. Finally, the importance of eosinophils in lung fibrosis was examined in IL-5 gene knockout mice (IL-5tm1Kopf). BLM treatment induced significant lung fibrosis in IL-5 knockout mice in the absence of eosinophilia. These findings indicate that eosinophils are not an absolute requirement for BLM-induced pulmonary fibrosis in the mouse.

Nitric oxide participates in the intestinal pathology associated with murine syngeneic graft-versus-host disease.

Syngeneic graft-versus-host disease (SGVHD) develops following lethal irradiation, reconstitution with syngeneic bone marrow, and treatment with a short course of cyclosporin A (CsA) therapy. The disease is characterized by the development of a T helper cell type 1-like cytokine response [interleukin (IL)-12, interferon-gamma (IFN-gamma), and tumor necrosis factor alpha], and macrophage activation is central to development of the syndrome. It has been shown that nitric oxide (NO) participates significantly in the development of allogeneic GVHD. Studies were initiated to determine if NO participates in the pathology associated with SGVHD. Significant increases in inducible NO synthase (iNOS) mRNA and circulating NO were found in the tissues of SGVHD versus control animals. Treatment of SGVHD animals with the iNOS inhibitor aminoguanidine (AG) reversed the pathology associated with this disease. Furthermore, AG treatment reduced the production of IL-12 and IFN-gamma mRNA in the colons of CsA-treated mice. These studies demonstrate that NO participates in the pathological processes that are associated with the development of murine SGVHD.

Induction of syngeneic graft-versus-host disease in LPS hyporesponsive C3H/HeJ mice.

Syngeneic GVHD (SGVHD) develops following syngeneic bone marrow transplantation and treatment with cyclosporine A. Previous studies have demonstrated a role for IL-12, IFN-gamma, and TNF-alpha in the development of murine SGVHD. Macrophages can be activated to secrete IL-12 and TNF-alpha via a T-cell-dependent or T-cell-independent pathway (LPS or bacterial products). Studies were designed to determine if LPS participated in the development of SGVHD in C3H/HeN (LPS-responsive) and C3H/HeJ (LPS-hyporesponsive) mice. C3H/HeJ and C3H/HeN mice had similar levels of disease induction and pathology. Following induction of SGVHD, treatment of C3H/HeN, but not C3H/HeJ, mice with a sublethal dose of LPS resulted in mortality. However, neutralization of IL-12 abrogated the development of disease in C3H/HeJ mice, demonstrating that activated macrophages and their products participated in the development of SGVHD in these animals. These data suggested that LPS responsiveness was not a predisposing factor for SGVHD induction.

Falsely incompatible B-cell flow cytometry crossmatch after pronase treatment: a case report.

This report presents a falsely incompatible B cell crossmatch by flow cytometry in a lung transplant recipient. The patient was a 35-year-old Caucasian male with end-stage lung disease secondary to cystic fibrosis whose pretransplantation serologic workup did not disclose the presence of anti-HLA class II antibodies by single antigen bead testing. Unexpectedly, crossmatch of recipient sera with pronase-treated donor lymphocytes resulted in antibody binding to B cells only. The positive reactivity was reproducible in pronase-treated autologous B cells. Recipient sera did not react with nontreated donor or autologous lymphocytes. Herein, we describe our approach to this unexpected crossmatch result and consider the implications of false-positive crossmatch results on transplantation.

The effect of human IL-2-activated natural killer and T cells on graft-versus-host disease and graft-versus-leukemia in SCID mice bearing human leukemic cells.

We have previously reported that xenogeneic lethal acute graft-versus-host disease (GVHD) was induced by transplantation of a mixture of human IL-2 activated natural killer (NK) and T cells into SCID mice conditioned with 4 Gy total-body irradiation (TBI), but not by IL-2-activated pure human T cells or NK cells. TBI and transplantation of the mixture of activated cells were both required to produce the lethal effect. We now report the effect of human IL-2 activated NK, T, or NK+T effector cells on the development of acute and chronic GVHD and GVL in SCID mice bearing human leukemic cells. Ten days after being inoculated i.v. with 2 x 10(7) human U-937 or K-562 leukemic cells, SCID mice, hereafter termed hu-leukemic mice, were radiated with 4 Gy TBI and transplanted i.v. with 5 x 10(7) human IL-2-activated NK, T, or NK+T effector cells. Hu-leukemic control mice received neither TBI nor effector cell transplantation. Acute GVHD-positive control SCID mice were transplanted with 5 x 10(7) H-2-incompatible C57Bl/6 splenocytes following 4 Gy TBI. The mice were observed for signs of GVHD and leukemia for 90 days. Twenty of 20 non-effector cell-transplanted control hu-leukemic mice developed signs related to leukemia and died with leukemic infiltration in the brain, liver, kidney, and lung 50-65 days after inoculation. Flow cytometry (FC) demonstrated 21-89% human leukemic cell infiltration in the bone marrow. Fourteen of 14 hu-leukemic mice transplanted with NK+T effector cells did not develop signs of advanced leukemia but died within 17 days of acute GVHD. FC demonstrated no human leukemic cells in their marrow. Twelve of 15 and 18 of 25 hu-leukemic mice transplanted with either NK or T cells survived 90 days without any evidence of symptomatic leukemia (P < 0.01 compared with non-effector cell-transplanted groups). NK-transplanted hu-leukemic animals experienced mild-to-moderate acute GVHD during the first 10-20 days posttransplantation, but gradually recovered and did not develop chronic GVHD. Hu-leukemic animals transplanted with T effector cells manifested no signs of leukemia or acute GVHD but chronic GVHD skin lesions appeared 80-90 days after transplantation. We conclude that acute GVHD, chronic GVHD, and GVL are associated but separable phenomena.

Thy1+ bone marrow cells regulate the induction of murine syngeneic graft-versus-host disease.

A syngeneic graft-versus-host disease (GVHD)-like syndrome has been shown to be inducible in some strains of mice after lethal irradiation, reconstitution with syngeneic bone marrow (BM), and treatment with a short course of CsA therapy. Since Thy1+ BM cells have been shown to regulate the development of other experimental autoimmune diseases, it was important to determine their role in the inducibility of syngeneic GVHD (SGVHD) in different strains of mice. Lethally irradiated mice were reconstituted with either syngeneic BM or T cell-depleted syngeneic BM, then treated with CsA or diluent. Removal of Thy1+ cells from BM before reconstitution of an inducible strain, C3H/HeN, exacerbated SGVHD when compared with animals given whole BM cells before CsA treatment. Furthermore, a noninducible strain, C57BL/6 mice, developed SGVHD when reconstituted with T cell-depleted syngeneic BM but not BM before CsA therapy. These results suggest that Thy1+ BM cells may regulate the development of SGVHD, and be of importance in controlling autoreactivity after bone marrow transplantation.

Acute graft-versus-host-like disease induced by transplantation of human activated natural killer cells into SCID mice.

Whereas T lymphocytes are essential for the initiation of acute graft-versus-host disease (aGVHD), it is not at all clear whether they or other cells or noncellular factors actually mediate the characteristic lesions. This report describes the in vivo effects of human NK cells, T cells, and cytokines on the induction of aGVHD in 4 Gy sublethally irradiated C.B-17 scid/scid (SCID) mice. Human NK and T lymphocytes were obtained separately by antibody- and complement-mediated negative selection from the peripheral blood of normal donors and expanded in medium containing rIL-2 and irradiated autologous feeder cells. The characteristics of the two groups of cells were analyzed before injection into SCID mice. Cytofluorometric phenotyping demonstrated that 70-95% of NK-enriched cells expressed CD3-, CD16+, CD56+, and CD8-dim+; ninety-seven per cent of T cells expressed CD3+, TCR-alpha/beta+, CD4+, or CD8-bright+. Analysis of K562 and Daudi cultured target cell lines demonstrated 40-50% higher cytotoxicity by NK-enriched cells as compared with activated T lymphocytes. TNF-alpha cytokine production was greatly increased in activated NK cells (250 pg/ml) as compared with T cells (25 pg/ml) and fresh PBMC (12.5 pg/ml). IFN-gamma was increased in both NK and T cells. After i.v. injection of 1-5 x 10(7) cells into irradiated SCID mice, minor to severe skin lesions, diarrhea, and weight loss occurred in NK- but not the T cell-injected animals. In NK-injected animals, thinning and focal loss of epithelium with pyknotic nuclear change and degeneration and loss of skin appendages were observed. Single cell necrosis, crypt abscess formation, and loss of glandular epithelium developed in the colon of NK but not in T cell-injected animals. These findings are very similar to allogeneic aGVHD in SCID mice injected with C57BL/6 mouse splenocytes. Immunohistological staining with anti-human CD56, CD3, TNF-alpha, and IFN-gamma antibodies demonstrated CD56+ cells in association with TNF-alpha and IFN-gamma secretion in the bowel of NK-injected animals. CD3+ cells were not found in the same tissues. These findings were not observed in T cell-injected and control mice. In summary, aGVHD-like lesions were induced by transplantation of xenogeneic human activated NK cells into SCID mice. We hypothesize that cytokines released from human NK cells play a central role in the pathogenesis of clinical aGVHD.

Induction of a syngeneic graft-versus-host disease-like syndrome in DBA/2 mice.

Syngeneic graft-versus-host disease has been shown to occur in syngeneic rat radiation chimeras after treatment with a short course of CsA. However, data concerning this model have been controversial in murine systems. We have successfully induced a GVHD-like syndrome in syngeneic mouse radiation chimeras treated transiently with CsA. Lethally irradiated (950 rads) DBA/2 mice were reconstituted with syngeneic bone marrow and treated daily, i.p. with 15 mg/kg CsA in olive oil for 21 days. Within 1 week after discontinuing CsA, animals developed clinical signs of GVHD including runting, hunched posture, and severe diarrhea. This disease was fatal for greater than 80% of treated animals within 4 weeks after cessation of CsA. Furthermore, the induction of syngeneic GVHD did not appear to be linked to a particular MHC haplotype. Histologically, there was pronounced lymphoid atrophy of the spleen and thymus. Sections of large intestine showed an acute inflammatory process involving the mucosal layer ranging from single-cell destruction to complete mucosal ulceration. This murine model of GVHD should provide new opportunities for studying the development and regulation of autoimmune processes.

In vivo reactivity of T cell clones isolated from mice with syngeneic graft-versus-host disease.

Syngeneic graft-versus-host disease (SGVHD) has been shown to occur in murine syngeneic radiation bone marrow chimeras following a short course of cyclosporine. To analyze the effector mechanisms present in diseased animals, four T cell clones (1D5, 1D8, 1C10, 2D8) were isolated from the spleens of C3H/HeN mice late in the disease course by cloning on irradiated syngeneic spleen cells. These clones were CD4+, alpha beta TCR+ and responded to I-Kk in vitro. In addition to I-Ek reactivity, three of the clones exhibited crossreactivity with the superantigen MIs 1a (mtv 7). Clones 1D5 and 1C10 were found to express TCR V beta chains (V beta 4 and V beta 8.1, respectively), which are normally present in the T cell repertoire of C3H/HeN mice. All SGVHD clones were found to be autoreactive in that they responded to syngeneic stimulator cells in the absence of xenogeneic serum proteins. To test in vivo activity, the 1D5 SGVHD clone was injected into the hind footpad of mice where it was shown to induce footpad swelling in a cell dose-dependent, I-Ek-specific manner in sublethally irradiated, but not normal mice. Histological analysis indicated that the clone induced dermal and subcutaneous edema that correlated directly with injection of 1D5 and not the control clone. Preliminary experiments suggested that the other three autoreactive clones behaved in a similar manner. These data are consistent with the involvement of a self-class II-specific CD4+ T cell in murine SGVHD.

Prospective DR matching for first cadaver donor renal allografts and retransplantation.

From January 1981 through 1983 80 cadaver donor renal allografts were transplanted at a single center utilizing prospective HLA-DR matching. All patients received at least two blood transfusions prior to transplantation. One year actual allograft survival of 77% for initial grafts and 57% for retransplantation was observed. When there was no DR mismatch the results were 84% and 80% respectively. Only 6% of no-DR-mismatch initial grafts were lost to rejection or patient death. These significantly better results were associated with decreased incidence of acute rejection episodes with transplants well matched for DR. Matching for A and B locus antigens conveyed no benefits in this series. Use of prospective DR matching for donor/recipient selection also resulted in efficient transplantation. Patients receiving initial grafts waited an average of 3.9 months while retransplanted patients waited an average of 13.5 months after being entered on the waiting list. The data suggest that if all transplant centers would preferentially share kidneys regionally on the basis of DR matching, nearly all patients could receive timely allografts with no DR mismatch and good results at one year with conventional immunosuppressive therapy.

Role of natural killer cells in the pathogenesis of human acute graft-versus-host disease.

Clinical acute graft-versus-host disease (aGVHD) was correlated with alterations in PBL phenotype and skin immunohistology in 52 patients transplanted with HLA-identical bone marrow. Concurrent with the emergence of aGVHD, there was a profound decrease in absolute number of CD3- T cells and an increase in CD3-CD16+, CD56+ (a subset of which coexpress CD8+ "dim") NK cells in the PBL. CD4+ T and CD20+ B lymphocytes failed to recover within 90 days in the patients with grades II-IV aGVHD. Ex vivo partial T cell depletion, in itself, did not significantly impair T cell recovery as compared to that in non-T-depleted recipients unless aGVHD occurred. Although leukocytic cellular infiltration in the skin was generally sparse, CD16+ NK lymphocytes were significantly increased in grades II-IV aGVHD. By contrast, there was no significant increase in CD3+, CD4+, or CD8+ lymphocytes in these lesions as compared to skin biopsies obtained from BMT patients without aGVHD or from normal skin. Taken together, these findings suggest that NK cells may be important in the pathogenesis of human aGVHD.

DNA cytometry in postirradiation cervical-vaginal smears.

Benign radiation change (BRC) in cervical-vaginal smears may be difficult to distinguish from postirradiation dysplasia (PRD) or recurrent cervical carcinoma. The utility of DNA analysis in postirradiation smears was evaluated retrospectively in 71 patients. Representative Papanicolaou smears were restained with a Feulgen method and 100 to 250 cells were analyzed for DNA content using the CAS 200 image analysis system. Thirty-three control irradiated patients had negative smears with a minimum 3-year follow-up. Thirty controls (91%) had diploid histograms with a mean coefficient of variation of 8.2% and an average of 6.8% of cells in S and G2/M phase. Three control patients had atypical nondiagnostic histograms. Twenty-three patients had abnormal smears and subsequent local recurrence; 21 (91%) had abnormal histograms, with seven showing polyploidy and 14 showing aneuploidy. The remaining 15 patients had abnormal smears diagnosed as PRD but no evidence of recurrent carcinoma. Five were polyploid, six were aneuploid, one was diploid, and three were atypical but nondiagnostic. Interactive DNA cytometry is useful in differentiating BRC from PRD and recurrent cancer. Aneuploidy is rarely, if ever, seen in negative smears with BRC. However, BRC may be associated with broad diploid peaks and increased proliferating cells. An abnormal histogram can be seen with PRD and does not always correlate with recurrent disease.

Enhanced graft-versus-host disease in older recipient mice following allogeneic bone marrow transplantation.

The incidence and severity of GVHD following bone marrow transplantation increases with recipient age. The role of recipient age on the development of GVHD was analyzed in a semi-allogeneic (C57BL/6-->(C57BL/6 x DBA/2)F1) murine GVHD system. Young adult (2 months) and old (12-14 months) recipient mice were lethally irradiated and reconstituted with young adult T cell-depleted bone marrow (ATBM) or ATBM and young spleen cells. A significantly higher percentage of old vs young recipients developed lethal GVHD. Furthermore, while pre-transplant conditioning with irradiation was not required to observe increased mortality in old recipients, irradiation predisposed the older animals for a more severe course of GVHD, suggesting that GVHD occurred in old compared to young animals in the absence of pre-transplant conditioning but was exacerbated by irradiation. Histologically, the immunological responses in the GVHD target organs were more severe in the old GVHD animals. In support of this observation, increased spontaneous proliferation was observed using lymphoid cells isolated from old vs young GVHD mice. These findings demonstrate that old recipients develop a more severe course of GVHD following BMT, and may present a unique opportunity to study age-related factors in the generation of GVHD.

Rejection of an MHC class II negative tumor following induction of murine syngeneic graft-versus-host disease.

Cyclosporin A (CsA) has been used clinically to induce graft-versus-host disease following autologous bone marrow transplantation in an attempt to destroy residual leukemia cells and reduce relapse. To analyze the antitumor potential of murine syngeneic graft-versus-host disease (SGVHD), C3H/HeN mice were lethally irradiated, reconstituted with T cell-depleted syngeneic bone marrow (ATBM) and treated with CsA for 21 days. Graft-versus-leukemia activity was assessed by challenging groups of olive oil-treated control ATBM (OO-ATBM) and CsA-treated (CsA-ATBM) mice 1 week after CsA therapy with graded doses of the syngeneic 38C13 B cell lymphoma. Following CsA treatment, up to 70% of CsA-ATBM developed SGVHD and more than 70% of the animals injected with 500 38C13 cells exhibited long-term survival (MST >80 days). In contrast, none of the OO-ATBM control mice developed SGVHD, and more than 75% of these mice died following injection of 500 38C13 tumor cells (MST = 34 days). Long-term survivors were not resistant to tumor challenge suggesting that tumor-specific immunity did not develop. Finally, class II negative 38C13 cells cultured in IL-4 or IL-10 were not inducible for MHC class II molecules, demonstrating that class II-independent antitumor mechanisms exist in SGVHD mice.

Aedes aegypti control in the Lao People's Democratic Republic, with reference to copepods.

An evaluation of the Lao Aedes aegypti control program and of the predatory abilities of copepods from Vientiane, Lao People's Democratic Republic was undertaken before a field release of copepods in Thongkankam village, Vientiane. Copepods were transported to Australia for evaluation of predatory abilities and their survival under various nutrient and pH conditions. Mesocyclops guangxiensis was chosen for release over M. aspericornis due to its higher reproduction rate and its ability to survive in lower nutrient environments. Mesocyclops guangxiensis was released into 142 containers and 20 wells in a village in Vientiane. Copepods were present in 7% of the containers after one month and were absent six months postinoculation. In comparison, 100% of wells were still positive after six months, with average numbers of Ae. aegypti in the wells decreasing from 59.5 +/- 18.5 (+/- SEM) to 0 after six months. Numbers of Culex quinquefasciatus and Anopheles maculatus also decreased to 0 after six months. This study indicates that predacious copepods will be accepted by the community and could be integrated as a low-cost, persistent control agent into new strategies for improving surveillance and control of dengue vectors.

Proliferating cell nuclear antigen in prostatic adenocarcinoma: correlation with established prognostic indicators.

OBJECTIVE: The utility of an antibody to proliferating cell nuclear antigen (PCNA), a growth-specific nuclear protein, was assessed as a prognostic variable for prostatic adenocarcinoma. Its expression was correlated with established prognostic indicators, including tumor grade, stage, prostatic-specific antigen (PSA), and percent of tumor in the gland at excision. METHODS: Forty archival needle biopsies containing a minimum of four hundred tumor cells were analyzed. Immunoperoxidase staining of paraffin sections was performed for PCNA (PC10) after pretreatment in antigen retrieval solution. A proliferative index (PI) for each case was derived using image analysis with measurement of at least four hundred twenty-five nuclei. RESULTS: PI values ranged from 2.4 to 31.3 percent. Mean PI values varied significantly (ANOVA, p = 0.005) among cases with dominant Gleason grade (DGG) of 3 (mean PI = 9.3%), 4 (mean PI = 13.7%), and 5 (mean PI = 18.8%). By t test, significant differences were noted for PI in cases with DGG 2 and 3 versus those with DGG 4 and 5 (p = 0.0065). PI for cases with DGG 3 versus 5 showed significant difference (p = 0.0017). Tumors of Gleason scores 5 to 7 differed significantly from those with scores 8 to 10 (p = 0.014). A statistical relationship for PI and PSA, clinical stage, and percent tumor at resection could not be established by linear regression. CONCLUSIONS: These findings suggest that additional study of the PI, as determined by PCNA immunohistochemistry and image analysis, may be warranted to determine its usefulness as an adjunctive parameter in prostate adenocarcinoma. This technique may be particularly useful in needle biopsies where limited tumor may render assessment of grade difficult.

Corneal allograft rejection following immunization.

Five patients developed corneal allograft rejection after immunization. One patient, a 33-year-old woman, received a tetanus toxoid booster nine months after a corneal transplant for keratoconus. Within four days she developed a graft rejection that required a penetrating keratoplasty two years later. Six months later, after hepatitis B immunization, the patient reported decreased vision and the graft was cloudy, but visual acuity was 20/20. The other four patients developed graft rejection after influenza immunization. Two of these four graft rejection episodes were successfully treated with high-dose corticosteroid therapy; all episodes occurred within several weeks of influenza immunization. Patients should be prudently counseled regarding the possible risks of immunization to corneal allograft survival.

Suprasellar germ cell tumor with extracranial metastases.

A case of metastasizing suprasellar germ cell tumor is described. Serum tumor markers were measured and the immunochemistry of the tumor was studied. The rarity of extracranial metastasis from suprasellar germinomas is noted, and the relevance to mixed tumors is discussed. The difficulties associated with small biopsies and the importance of tumor marker studies in all cases of an intracranial germ cell tumor are emphasized by the case study.

Enhancement or modulation of the vector competence of Ochlerotatus vigilax (Diptera: Culicidae) for ross river virus by temperature.

Two different doses of Ross River virus (RR) were fed to Ochlerotatus vigilax (Skuse), the primary coastal vector in Australia; and blood engorged females were held at different temperatures up to 35 d. After ingesting 10(4.3) CCID50/mosquito, mosquitoes reared at 18 and 25 degrees C (and held at the same temperature) had higher body remnant and head and salivary gland titers than those held at 32 degrees C. although infection rates were comparable. At 18, 25, and 32 degrees C, respectively, virus was first detected in the salivary glands on days 3, 2, and 3. Based on a previously demonstrated 98.7% concordance between salivary gland infection and transmission, the extrinsic incubation periods were estimated as 5, 4, and 3 d, respectively, for these three temperatures. When Oc. vigilax reared at 18, 25, or 32 degrees C were fed a lower dosage of 10(3.3) CCID50 RR/mosquito, and assayed after 7 d extrinsic incubation at these (or combinations of these) temperatures, infection rates and titers were similar. However, by 14 d, infection rates and titers of those reared and held at 18 and 32 degrees C were significantly higher and lower, respectively. However, this process was reversible when the moderate 25 degrees C was involved, and intermediate infection rates and titers resulted. These data indicate that for the strains of RR and Oc. vigilax used, rearing temperature is unimportant to vector competence in the field, and that ambient temperature variations will modulate or enhance detectable infection rates only after 7 d extrinsic incubation. Because of the short duration of extrinsic incubation, however, this will do little to influence RR epidemiology, because by this time some Oc. vigilax could be seeking their third blood meal, the latter two being infectious.

Spontaneous and ionizing radiation induced mutations involve large events when selecting for loss of an autosomal locus.

The mouse P19H22 embryonal carcinoma cell line contains two distinct chromosome 8 homologs, one derived from Mus musculus domesticus (M. domesticus) and the other derived from Mus musculus musculus (M. musculus). It also contains a deletion for the M. musculus aprt allele, which is located on chromosome 8. In this study, cells with spontaneous or induced aprt deficiencies were isolated from P19H22 and examined to determine the nature of the mutational events that had occurred. Ultraviolet radiation (UV), ethyl methanesulfonate (EMS), and two forms of ionizing radiation, 137Cs and 252Cf, were used for mutation induction. DNA preparations from the aprt deficient cells were initially screened with a Southern blot analysis and separated into two broad classes: those that had lost the M. domesticus aprt allele and those that had retained it. The overwhelming majority (> 95%) of the spontaneous and ionizing radiation-induced mutants exhibited aprt gene loss, indicating that relatively large events had occurred and that homozygosity for the deleted region was not a lethal event. Loss of heterozygosity for syntenic markers was found to be a common event in cells exhibiting aprt gene loss. In contrast, a majority of the UV-induced mutants (61%) and a substantial minority of the EMS-induced mutants (38%) retained the aprt gene. A sequence analysis confirmed that base-pair substitutions were responsible for this class of mutation. Gene inactivation associated with hypermethylation of the promoter region was found to be a rare event and was not induced by any of the mutagenic agents tested. The results demonstrate the suitability of the P19H22 cell line for mutational studies, particularly those that are large in nature.