Effects of knockout of the protein kinase C beta gene on glucose transport and glucose homeostasis.

The beta-isoform of protein kinase C (PKC) has paradoxically been suggested to be important for both insulin action and insulin resistance as well as for contributing to the pathogenesis of diabetic complications. Presently, we evaluated the effects of knockout of the PKCbeta gene on overall glucose homeostasis and insulin regulation of glucose transport. To evaluate subtle differences in glucose homeostasis in vivo, knockout mice were extensively backcrossed in C57BL/6 mice to diminish genetic differences other than the absence of the PKCbeta gene. PKCbeta-/- knockout offspring obtained through this backcrossing had 10% lower blood glucose levels than those observed in PKCbeta+/+ wild-type offspring in both the fasting state and 30 min after i.p. injection of glucose despite having similar or slightly lower serum insulin levels. Also, compared with commercially obtained C57BL/6-129/SV hybrid control mice, serum glucose levels were similar, and serum insulin levels were similar or slightly lower, in C57BL/6-129/SV hybrid PKCbeta knockout mice in fasting and fed states and after i.p. glucose administration. In keeping with a tendency for slightly lower serum glucose and/or insulin levels in PKCbeta knockout mice, insulin-stimulated 2-deoxyglucose (2-DOG) uptake was enhanced by 50-100% in isolated adipocytes; basal and insulin-stimulated epitope-tagged GLUT4 translocations in adipocytes were increased by 41% and 27%, respectively; and basal 2-DOG uptake was mildly increased by 20-25% in soleus muscles incubated in vitro. The reason for increased 2-DOG uptake and/or GLUT4 translocation in these tissues was uncertain, as there were no significant alterations in phosphatidylinositol 3-kinase activity or activation or in levels of GLUT1 or GLUT4 glucose transporters or other PKC isoforms. On the other hand, increases in 2-DOG uptake may have been partly caused by the loss of PKCbeta1, rather than PKCbeta2, as transient expression of PKCbeta1 selectively inhibited insulin-stimulated translocation of epitope-tagged GLUT4 in adipocytes prepared from PKCbeta knockout mice. Our findings suggest that 1) PKCbeta is not required for insulin-stimulated glucose transport; 2) overall glucose homeostasis in vivo is mildly enhanced by knockout of the PKCbeta gene; 3) glucose transport is increased in some tissues in PKCbeta knockout mice; and 4) increased glucose transport may be partly due to loss of PKCbeta1, which negatively modulates insulin-stimulated GLUT4 translocation.     

IL-4 dependent resistance to the tapeworm Mesocestoides corti (Cestoda) in mice

Three-week Mesocestoides corti infections of C57BL/6 mice showed significantly raised parasite counts in animals with a targeted knockout of the IL-4 gene. By contrast, antibody neutralization of IL-5 and inhibition of eosinophilia had no effect on parasite numbers. In SV/129 mice, knockout of the interferon gamma receptor and inducible nitric oxide synthase genes had no significant effect on parasite counts. In IL-4(-/-)mice the dominant IgG1 antibody response was dramatically reduced, with a concomitant increase in IgG2a/b responses and a partial twofold reduction in IgM and IgE responses. We conclude that murine resistance to M. corti is dependent on IL-4, but occurs independently of IL-5 and eosinophils. This provides the first direct evidence for IL-4 mediated immunity of mice to infection with a tissue-dwelling platyhelminth.     

The Outcome of Coxsackievirus B3-(CVB3-) Induced Myocarditis is Influenced by the Cellular Immune Status

Mice develop a marked age-related susceptibility to myocardial coxsackievirus B3 (CVB3) infections. The lesions observed in mice resemble closely those seen in the human disease. Experimental murine models of CVB3-induced myocarditis have shown that both, host and viral genetic factors, can influence susceptibility to the infection as well as the persistence and progression of the disease. Recently, we have shown that CD4 T cell-deficient MHC Class II knockout mice develop a strong fibrosis with virus persistence in the heart tissue and without production of neutralizing antibodies. To examine the role of CD4+ T cells and especially the role of the T helper 1 cell response for the outcome and pathogenesis of CVB3-induced myocarditis in more detail, 2 different mouse strains with identical genetic background (H-2b) were infected with CVB3-Mü/J (Nancy strain). Immunocompetent C57BL/6 mice and mice with targeted disruption of interleukin (IL-)4 gene (IL-4-/- mice) developed a severe acute myocarditis on day 7 post infection (p.i.). The CVB3-induced inflammation was cured until the 21st day p.i. in hearts of C57BL/6 mice. IL-4-/- mice with insufficient T helper-2 cell immune response developed a severe myocardial damage between day 7 and 21 p.i. with prolonged virus persistence in the heart tissue. Therefore, we suggest that despite an obvious normal T helper-1 cell cytokine pattern, IL-4-/- mice are more susceptible to long-term heart muscle injuries after infection with CVB3.     

Differential effect of inbred mouse strain (C57BL/6, DBA/2, 129T2) on insulin secretory function in response to a high fat diet

The increasing production of genetically-modified mouse models has necessitated studies to determine the inherent physiological characteristics of commonly used mouse strains. In this study we examined insulin secretory function in response to an intravenous bolus of glucose or glucose plus arginine in anesthetized C57BL/6, DBA/2 and 129T2 mice fed either a control or high fat diet for 6 weeks. The results show that 129T2 mice had higher fasting plasma glucose levels and lower fasting plasma insulin levels compared with C57BL/6 and DBA/2 mice regardless of diet. Furthermore, 129T2 mice were glucose intolerant and secreted significantly less insulin in response to glucose and glucose plus arginine irrespective of diet compared with the other two strains of mice. DBA/2 mice hypersecreted insulin in response to glucose and glucose plus arginine compared with C57BL/6 and 129T2 mice. Moreover while first phase insulin secretion was appropriately increased in response to the high fat diet in C57BL/6 and 129T2 mice, this was not the case for DBA/2 mice. Mean islet area was decreased in response to a high fat diet in DBA/2 mice, while there was no dietary effect on the other two strains. This study highlights the inherent genetic differences that exist among seemingly normal strains of mice that are commonly used to make transgenic and knockout mice. Understanding these differences will provide researchers with the information to choose the appropriate genetic background on which to express their particular genetic alteration.     

Analysis of metallothionein brain gene expression in relation to ethanol preference in mice using cosegregation and gene knockouts

BACKGROUND: Metallothioneins (MTs) are ubiquitously expressed intracellular proteins that bind heavy metals and are involved in cytoprotection against several types of stress agents including chemicals, hormones, and oxidants. We have previously reported 1 isoform, MT-II, as a possible candidate gene for ethanol (EtOH) preference (EP) determination in mice. METHODS: Semiquantitative RT-PCR was used to determine brain mRNA levels of MT-I and MT-III in 4 inbred mouse strains with variable EP. Following this, cosegregation of MT-II brain expression with EP was analyzed in F2 mice from 2 intercrosses (C57BL/6J x BALB/cJ and C57BL/6J x DBA/2J). Studies on MT-I/MT-II knockout (KO) mice were also undertaken to further explore this relationship. RESULTS: Our results suggest that MT-I is responsive to EtOH, with no evidence of basal-level differences between strains. Conversely, MT-III shows no EtOH response, yet indicates a possible strain-specific feature with C57BL/6J having the lowest levels of brain MT-III. Metallothionein-II expression cosegregates with EP in F2 mice from a C57BL/6J (preferring) and DBA/2J (avoiding) intercross. Although F2 mice from a cross with C57BL/6J and BALB/cJ (avoiding) strains follow a similar pattern, the results are not statistically significant. Metallothionein-I/MT-II knockout (MT-KO) mice appear to have smaller litter sizes as well as higher weight compared with controls (129S1/SvImJ) and also show a slight increase in EP. CONCLUSIONS: Metallothionein-II remains the primary candidate of the mouse MT gene family for involvement in EP. Its effect on EP appears to be dependent on the genetic background. Such conclusions are based on results from C57BL/6J, BALB/cJ, DBA/2J, and 129 inbred mouse strains. Evidence also points to shared neural pathways involved in weight gain and obesity. The complex interactions between MT-II, EP, and weight gain/obesity remain to be studied.     

High doses of digitalis increase the myocardial production of proinflammatory cytokines and worsen myocardial injury in viral myocarditis: a possible mechanism of digitalis toxicity

Results of recent studies suggest that proinflammatory cytokines cause myocardial contractile dysfunction, and that the drugs used to treat heart failure modulate the production of cytokines. This study was designed to examine the effects of digoxin in a murine model of heart failure induced by viral myocarditis. Four-week-old inbred DBA/2 mice were inoculated intraperitoneally with encephalomyocarditis virus (EMCV). Digoxin was given orally in doses of 0.1, 1 or 10 mg/kg daily from the day of virus inoculation. Interleukin (IL)-1beta, IL-6 and tumor necrosis factor (TNF)-alpha production in the heart were measured on day 5 after EMCV inoculation by enzyme-linked immunosorbent assay. The 14-day mortality tended to be increased in mice treated with 1 mg/kg, and was significantly increased in the group treated with 10 mg/kg per day. Myocardial necrosis and cellular infiltration on day 6 were significantly more severe in the high-dose digoxin group than in the control group. In the animals treated with 1 mg/kg digoxin, IL-1beta was significantly higher than in the control group. Intracardiac TNF-alpha levels were increased in a dose-dependent manner. These results suggest that digoxin worsens viral myocarditis, and that its use in high doses should be avoided in patients suffering from heart failure due to viral myocarditis.     

Difference in thioredoxin expression in viral myocarditis in inbred strains of mice.

Redox regulating mechanisms may be involved in the pathogenesis of viral myocarditis and thioredoxin (TRX) is a small multifunctional protein that contains a redox active sequence. The present study investigated the histopathology and characteristics of TRX expression in acute coxsackievirus B3 myocarditis in inbred strains of mice (severe myocarditis in DBA/2 mice, moderate myocarditis in BALB/c mice and mild myocarditis in C57BL/6 mice). Thioredoxin was upregulated and its expression correlated with the severity of the disease. In addition, 8-hydroxy-2'-deoxyguanosine, which is an established marker for oxidative stress, was concominantly positive in damaged myocytes. Thus, TRX may be specifically induced by the acute inflammatory stimuli in murine viral myocarditis, and the severity and development of acute viral myocarditis may be regulated by the cellular redox state.     

Natural resistance of 129/SvJ mice to Strongyloides venezuelensis infection

The susceptibility of 129/SvJ mice to infection with Strongyloides venezuelensis was compared with that of C57BL/6 mice. After a primary infection, daily egg output in faeces (EPG) from 129/SvJ mice was lower and terminated earlier than that from C57BL/6 mice. Adult worm recovery from the small intestine of 129/SvJ mice on day 7 was also lower than that of C57BL/6 mice. When the numbers of larvae recovered from the lungs were examined on days 2, 3 and 4 after a primary infection, they were comparable between the two strains. On the other hand, when an equal number of larvae recovered from the lungs of each strain on day 3 were implanted orally into homologous strain mice, the magnitude of EPG and the number of adult worms in the small intestine on day 5 after implantation were significantly lower in 129/SvJ than in C57BL/6 mice. The number of mucosal mast cells in the jejunum was not significantly different between 129/SvJ and C57BL/6 naive mice. Total chondroitin sulphate concentration in the gut washings obtained from naive mice was significantly higher in 129/SvJ (11.34 +/- 9.48) than in C57BL/6 mice (1.09 +/- 0.77, P < 0.05). These results indicate that the natural resistance of 129SvJ mice to S. venezuelensis infection is expressed at the intestine, probably due to higher concentration of chondroitin sulphate, which prevents establishment of S. venezuelensis.     

Encephalomyocarditis virus myocarditis in inbred strains of mice--chronic stage

Inbred strains of A/J, BALB/c, C3H/He, C57BL/6 and DBA/2 mice were inoculated with the M variant of encephalomyocarditis virus having a titer of 100 TCID50/0.1 ml. Myocardial lesions were seen in 73 of 150 BALB/c mice (48.7%), 160 of 259 C3H/He mice (61.8%) and 115 of 174 DBA/2 mice (66.1%). No pathologic findings were noted in A/J and C57BL/6 mice. In C3H/He and DBA/2 mice, dilatation and hypertrophy of the heart accompanying myocardial lesions persisted up to the 8th month after virus inoculation. The present study revealed that myocardial lesions similar to those in congestive (dilated) cardiomyopathy persisted for a long period after viral infection.     

Overexpression of interleukin-6 aggravates viral myocarditis: impaired increase in tumor necrosis factor-alpha

The process of inflammation and immune response is regulated by proinflammatory cytokines. Interleukin-6 (IL-6), one of the proinflammatory cytokines, plays a potentially critical role in viral-induced myocarditis. Our previous work demonstrates that exogenous IL-6 administration, given at the time of encephalomyocarditis virus (EMCV) inoculation in C3H/HeJ mice, has a protective effect on myocardium and improves survival rates. In the present study, we examined whether overexpression of IL-6 modified viral myocarditis. On day 3 and 10 after inoculation with EMCV, the ratio of heart weight to body weight and myocardial injury were significantly increased in IL-6 transgenic mice (IL-6TG). On day 3, a reduction of viral clearance was shown by the presence of elevated viral titers and viral replication in the heart of IL-6TG. The concentrations of serum tumor necrosis factor- alpha (TNF alpha) were dramatically increased in wild-type mice on day 1, in contrast, this change was not observed in IL-6TG. Treatment with recombinant human TNF (2 microg) significantly improved viral clearance in the IL-6TG hearts. Thus, overexpression of IL-6 promotes myocardial injury by interrupting both the cytokine network and viral clearance. These experiments suggest the possibility that IL-6 is one of the factors that accelerates tissue damage, including myocardial injury, in the viral myocarditis.     

Role of substance P in viral myocarditis in mice

Substance P (SP) is widely expressed in the central nervous system and in peripheral tissues such as myocardial nerves. We examined SP in viral myocarditis in mice induced by encephalomyocarditis virus (EMCV). Localization of SP in the hearts was examined immunohistochemically, and concentrations of SP in hearts and sera were measured by enzyme immunoassay. Substance P levels and density of SP-containing cells in murine hearts on day 6 after EMCV inoculation were decreased compared with those in normal controls. There was a negative correlation between SP levels in the hearts and ratio of heart weight to body weight of the mice at 6 days. Circulating SP levels were decreased in mice on day 6 after EMCV inoculation, and further decreased on day 14. Substance P in hearts and sera is decreased in viral myocarditis in mice, suggesting that SP may play a role in the pathogenesis of viral myocarditis, and that interaction of the neuropeptide nervous system and mast-cell immune system is important in the pathogenesis of viral myocarditis.     

Role of type 1 T helper cells in the resolution of acute Streptococcus pneumoniae sinusitis: a mouse model

BACKGROUND: We examined the importance of the adaptive and innate immune responses in the resolution of an acute bacterial sinus infection in mice. Methods: Recombinase-activating gene knockout (RAG-1(-/-)) (no lymphocytes) and C57BL/6 (wild-type) mice were infected with Streptococcus pneumoniae. For determination of the cell type involved, lymphocytes from mice were adoptively transferred into RAG-1(-/-), C57BL/6 (all lymphocytes), B cell-deficient, and T cell-deficient mice. The degree of infection and inflammation was determined by quantification of S. pneumoniae from nasal lavage and analysis of sinus tissue, respectively. RESULTS: In C57BL/6 mice, both the infection and inflammation resolved in 21 days, whereas neither resolved in RAG-1(-/-) mice. When C57BL/6 lymphocytes were adoptively transferred into RAG-1(-/-) mice, resolution of the infection and inflammation occurred. Mice without B cells were able to clear the infection, whereas mice without T cells could not clear it. In vitro stimulation of the draining lymph nodes of the infected mice by use of heat-killed S. pneumoniae led to the production of interferon (IFN)- gamma. Flow-cytometric analysis of lymphocytes obtained from sinus mucosa and draining lymph nodes showed an increase in the number of type 1 T helper cell-like cells over that in control mice. CONCLUSIONS: RAG-1(-/-) mice with innate immunity but no lymphocytes contain--but cannot clear--a bacterial sinus infection. Lymphocytes transferred to RAG-1(-/-) mice clear the infection. The sinus mucosa and draining lymph nodes show an increase in T cells generating IFN- gamma. These data demonstrate that T cells are essential in clearing an acute S. pneumoniae bacterial sinus infection.     

Portal system peculiarities may contribute to resistance against schistosomes in 129/J mice

Using the technique of latex microsphere injection into a mesenteric vein, evidence has been obtained for a 'leaky' portal system in 129/J mice maintained in this institute (WEHI 129/J) and also in C57BL/6 mice. Thus, 20-microns beads injected into the portal venous system of BALB/c and (BALB/cx129/J)F1 mice are trapped efficiently in the liver whereas in many 129/J and C57BL/6 mice the bulk of the injected beads is found in the lungs. Peculiarities in the hepatic portal system may thus contribute to resistance of WEHI 129/J mice (and to a lesser extent C57BL/6 mice) to infection with Schistosoma mansoni and S. japonicum. The possibility is raised by the data that increased access to, or residency times in, the lungs increases opportunities for expression of anti-schistosome immune responses and, in particular, the effects of recently described T-cell-initiated inflammatory responses in this organ. The variability within groups of WEHI 129/J and C57BL/6 mice in susceptibility to schistosomes as well as hepatic trapping of injected microspheres suggests that an infection of unknown type, proposed by others, contributes to the several peculiarities that have been described in at least 129/J mice.     

Spontaneous occurrence of autoimmune cholangitis in senescent mice

Abstract The biliary lesions that developed spontaneously in senescent female C57BL/6NCrj mice were investigated. Degeneration of the bile duct epithelium was observed in 12 of 13 mice (92%), destruction of bile duct epithelial cells was seen in six of 13 mice (46%) and chronic non-suppurative destructive cholangitis was found in three of 13 mice (23%). The biliary lesions were characterized by prominent round cell infiltrates in the portal areas. Extra-hepatic lesions such as sialoadenitis were observed in six mice (46%) and pancreatitis in seven (53%). IgM class antipyruvate dehydrogenase antibody was positive in one of three C57BL/6NCrj mice not given anti-Lyt 2 antibody and in three of six C57BL/6NCrj mice injected with anti-Lyt 2 antibody. These lesions were not observed in male C57BL/6NCrj mice, young female C57BL/6NCrj mice, or ICR mice. However, by transferring the splenic cells of senescent female C57BL/6NCrj mice to 6 week old females, the biliary lesions could be transferred at the rate of 6/9. The lymphocytes infiltrating in the bile ducts were CD8 positive lymphocytes. Moreover, in the ultrastructural immunocytochemical analysis of lymphocytes infiltrating bile duct epithelia, CD8 positive lymphocytes often formed broad contacts with the epithelial cells. The biliary lesions developing spontaneously in these mice are similar to those found in human primary biliary cirrhosis.     

Control of hemopoiesis in mice by sensitized L3T4+ Lyt2-lymphocytes during infection with bacillus Calmette-Guérin.

When injected intravenously with bacillus Calmette-Guérin (BCG; 10(7) viable units), C57BL/6 mice rapidly develop a transient anemia associated with an increased number of granulocytes and monocytes, whereas C3H/He mice do not. Because these two features are lacking in C57BL/6 nude mice we postulated that T lymphocytes can regulate hemopoiesis during infection. To assess further the role in hemopoiesis of T lymphocytes present in bone marrow of C57BL/6 and C3H/He mice, the frequency of BCG-specific T lymphocytes and their surface marker phenotype were determined by limiting dilution analysis and use of monoclonal antibodies. The number of BCG-specific T lymphocytes was estimated to be 50- to 100-fold higher in bone marrow of C57BL/6 than in that of C3H/He mice. Although L3T4+ Lyt2-and L3T4- Lyt2+ BCG-specific T lymphocytes were generated in mice of both strains, in C57BL/6 mice L3T4+ cells were induced preferentially from day 1 through day 5 after infection in correlation with hemopoietic changes. The relation between T-cell immune response and hemopoietic changes was substantiated by results obtained after in vivo treatment with monoclonal antibodies. Selective depletion of L3T4+ T cells by in vivo injection of anti-L3T4 monoclonal antibodies (GK 1-5) inhibited the development of the anemia and the related increased production of phagocytes in C57BL/6 mice receiving BCG.     

Disruption of the striated muscle glycogen-targeting subunit of protein phosphatase 1: influence of the genetic background

A prediabetic phenotype of glucose intolerance, insulin resistance and obesity was observed at approximately 12 months of age in mice homozygous for a null allele of the major skeletal muscle glycogen-targeting subunit G(M) of protein phosphatase 1 (PP1) and derived from a 129/Ola donor strain. In this study, backcrossing of these G(M)-/- mice (termed obese G(M)-/- mice) onto two different genetic backgrounds gave rise to lean, glucose-tolerant, insulin-sensitive G(M)-/- mice (termed lean G(M)-/- mice), indicating that at least one variant gene in the 129/Ola background, not present in the C57BL/6 or 129s2/sV background, is required for the development of the prediabetic phenotype of obese mice. Slightly elevated AMP-activated protein kinase alpha2 activity in the skeletal muscle of lean C57BL/6 mice was also observed to a lesser extent in the obese G(M)-/- mice. Normal or slightly raised in vivo glucose transport in lean C57BL/6 G(M)-/- mice compared with decreased glucose transport in the obese G(M)-/- mice supports the tenet that adequate transport of glucose may be a key factor in preventing the development of the prediabetic phenotype. The pH 6.8/pH 8.6 activity ratio of phosphorylase kinase was increased in lean C57BL/6 G(M)-/- mice compared with controls indicating that phosphorylase kinase is an in vivo substrate of PP1-G(M).     

Therapy with the nonpeptide endothelin receptor antagonist 97-139 in a murine model of congestive heart failure: reduction of cardiac mass and myofiber hypertrophy

Endothelin-1 (ET-1) is a potent vasoconstrictor. This peptide exerts numerous effects on the heart, including regulation of cardiomyocyte growth during hypertrophy. The effects of the structurally novel, nonpeptide, ET-1-selective, competitive antagonist (ETA) 97-139 were investigated in mice with congestive heart failure (CHF) and myocardial hypertrophy. Morphological and microscopical analyses were conducted on day 56 after viral inoculation following 28 day treatment with 99-139. Eight week-old DBA2 mice were intraperitoneally inoculated with encephalomyocarditis virus at a dose of 500 pfu/mouse. The 30 mice were divided into two groups--an ETA treated group and an untreated group. Heart weight (HW) in the infected group was significantly (p < 0.05) increased compared to that in the uninfected group. HW and the HW/body weight (BW) ratio were significantly (p < 0.05) reduced in the ETA treated group compared with the untreated group (HW; 127.7 +/- 6.2 mg vs 144.3 +/- 4.2 mg, HW/BW; 4.9 +/- 0.9 x 10(-3) vs 5.4 +/- 0.5 x 10(-3)). Myofiber diameter in the ETA treated group was significantly reduced compared with the untreated group (12.1 +/- 1.5 microm vs 14.3 +/- 1.9 microm). These results suggest the ET-1 receptor antagonist 97-139 has an effect on the reduction of cardiac mass and myofiber hypertrophy, and that 97-139 may be a useful agent for CHF due to viral myocarditis.     

Primary lung fibroblasts from the 129 mouse strain exhibit reduced growth factor responsiveness in vitro.

Lung fibroblasts are activated to proliferate and produce connective tissue during the development of lung fibrosis. The 129 mouse strain does not develop asbestos-induced fibrogenesis, whereas several other inbred strains rapidly respond to inhaled fibers. Thus, in the experiments presented here, we have compared the responses of primary lung fibroblasts isolated from 129 and C57BL/6 mice. The 129 and C57BL/6 mouse lung fibroblasts (MLFs) proliferated similarly in 10% fetal bovine serum (FBS), but after quiescence, the 129 MLFs grew more slowly in serum and responded less to the BB isoform of platelet-derived growth factor. This is consistent with our finding that the mRNA for the PDGF-a receptor exhibits reduced expression by the 129 MLFs compared to those from C57BL/6 mice. Fibroblasts from the SJL mouse strain, from a C57BL/6-129 hybrid, and from the 3T3 cell line all proliferated more vigorously than MLFs from the 129 mice. In addition, the 129 MLFs exhibited reduced expression of alpha1 procollagen mRNA consequent to treatment with tumor necrosisfactor alpha. Based on these new findings, we suggest that the reduced fibrogenesis in asbestos-exposed 129 mice is due to an intrinsic difference in the ability of the lung fibroblasts to respond to peptide growth factors.     

Genetic variability in the response of normal murine hematopoietic progenitor cells to photochemical purging

The photosensitizer, Merocyanine 540 (MC540) is currently undergoing phase I/II clinical evaluation as a purging agent for autologous stem cell grafts from patients with AML, ALL, CML, HD, and NHL. The preclinical data that provide the basis for these trials were mostly conducted in or with cells from B6D2F1 and B6AF1 mice. These experiments indicated that combinations of MC540 and light that deplete leukemia cells by 5–8 log deplete granulocyte/macrophage progenitors (CFU-GM) and the radio-protective capacity of bone marrow cells by about 1 log. We have recently shown that hematopoietic cells from 129 mice are substantially less sensitive to MC540-purging than cells from B6D2F1, B6AF1, and C57BL/6 mice. For instance, a combination of MC540 and light that depletes CFU-GM from C57BL/6mice 7.9 fold, depletes CFU-GM from 129 and B6129F1 mice merely 1.4-fold and 2-fold, respectively. This marked difference in photosensitivity cannot be explained by strain-specific differences in the ratio of photosensitive granulocye (CFU-G to photoresistant macrophage (CFU-M) progenitors, as the same rank order of sensitivity is also observed among unipotent CFU-G and CFU-GM as well as among erythroid progenitors from 129, B6129F1, and C57BL/6 mice.Flow cytometric binding studies show increased binding of MC540 to bone marrow cells from C57BL/6 mice, suggesting that the greater photosensitivity of C57BL/6 cells is at least in part attributable to a greater expression of MC540 binding sites in C57BL/6 marrow cells. These findings have obvious implications for the preclinical and clinical evaluation of merocyanines and related purging agents. They also suggest new experimental approches for investigations into the genetic control of photosensitivity.