Comparison of platelet antibody screen, crossmatching and HLA antibody testing in patients refractory to platelet transfusions.

Patients undergoing recurrent platelet transfusions can become refractory to these transfusions. Platelet antibody screens (Immucor), platelet crossmatching assays (Immucor), and HLA antibody testing are commonly used to test these patients. The relative effectiveness of these tests has not been determined. A higher incidence of strongly positive screen results that did not predict crossmatch results was anecdotally noted. Therefore, the results of the platelet antibody screens and crossmatches were systematically compiled over a 12-year period from 2010 to 2021. Of note, the Immucor Capture-P Ready Screen (platelet antibody) had a recall in March 2013 after which the performance of the test appears to have changed. The positivity rate of the platelet antibody screen increased over the course of the study, and this was statistically significant when analyzing year as a continuous variable and when grouping years by four-year periods (2010-13,2014-17,2018-21). In contrast, platelet crossmatch reactivity decreased slightly throughout this period. During the 2018-21 period, HLA antibody testing was commonly performed and correlated well with the crossmatch testing but not with the screen. These results suggest that the drastic increase in positivity we observed in the platelet antibody screen over this period is due to increased analytic sensitivity (with possible reduced specificity) of the screen and not a change in our patient population. Based on these results, the platelet antibody screen has little clinical utility and directly performing platelet crossmatching or HLA antibody testing is recommended for patients suspected to be refractory to platelet transfusions due to alloimmune-mediated factors.

Patient ABO blood type is a major predictor of a positive DAT following a transfusion reaction.

BACKGROUND: A direct antiglobulin test (DAT) checks for antibody or complement on the surface of RBCs and is often done following a transfusion reaction. While passive anti-A and anti-B antibodies are known to cause positive DATs, the extent this occurs following transfusion is unknown. STUDY DESIGN AND METHODS: DAT results, ABO type, eluate information, and blood product information were recorded on 1097 transfusion reactions at a large academic hospital over 8 years. The effect of patient blood type, product type, and plasma compatibility of blood product transfused on DAT results were determined. Statistical significance was determined using Chi-squared testing. RESULTS: Patient ABO blood type was a strong predictor of a positive DAT, with type O patients having 6.7% positive rate and non-O patients having a positive rate of 20.6% (p < .0001). Plasma compatibility of the product was a strong predictor of a positive DAT, with plasma compatible transfusions having a 9.4% positive rate while plasma incompatible transfusions were positive 44% of the time (p < .0001). Elution studies found that anti-A/B antibodies were the most common antibody identified. Platelets were more likely to be associated with a positive DAT when compared with RBC transfusions (p < .05). CONCLUSIONS: These results demonstrate the patient ABO type and plasma incompatibility are strong predictors of positive DAT results following a transfusion reaction. Anti-A and anti-B antibodies are estimated to account for about 50% of positive DATs in this study.

Limited Variation between SARS-CoV-2-Infected Individuals in Domain Specificity and Relative Potency of the Antibody Response against the Spike Glycoprotein.

The spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is arranged as a trimer on the virus surface, composed of three S1 and three S2 subunits. Infected and vaccinated individuals generate antibodies against spike, which can neutralize the virus. Most antibodies target the receptor-binding domain (RBD) and N-terminal domain (NTD) of S1; however, antibodies against other regions of spike have also been isolated. The interhost variability in domain specificity and relative neutralization efficacy of the antibodies is still poorly characterized. To this end, we tested serum and plasma samples collected from 85 coronavirus disease 2019 (COVID-19) convalescent subjects. Samples were analyzed using seven immunoassays that employ different domains, subunits, and oligomeric forms of spike to capture the antibodies. Samples were also tested for their neutralization of pseudovirus containing SARS-CoV-2 spike and of replication-competent SARS-CoV-2. While the total amount of anti-spike antibodies produced varied among convalescent subjects, we observed an unexpectedly fixed ratio of RBD- to NTD-targeting antibodies. The relative potency of the response (defined as the measured neutralization efficacy relative to the total level of spike-targeting antibodies) also exhibited limited variation between subjects and was not associated with the overall amount of antispike antibodies produced. These studies suggest that host-to-host variation in the polyclonal response elicited against SARS-CoV-2 spike in early pandemic subjects is primarily limited to the quantity of antibodies generated rather than their domain specificity or relative neutralization potency. IMPORTANCE Infection by SARS-CoV-2 elicits antibodies against various domains of the spike protein, including the RBD and NTD of subunit S1 and against subunit S2. The antibody responses of different infected individuals exhibit different efficacies to inactivate (neutralize) the virus. Here, we show that the observed variation in the neutralizing activity of the antibody responses in COVID-19 convalescent subjects is caused by differences in the amounts of antibodies rather than their recognition properties or the potency of their antiviral activity. These findings suggest that COVID-19 vaccine strategies that focus on enhancing the overall level of the antibodies will likely elicit a more uniformly efficacious protective response.

SARS-CoV-2 antibody changes in patients receiving COVID-19 convalescent plasma from normal and vaccinated donors.

Vaccination has been shown to stimulate remarkably high antibody levels in donors who have recovered from COVID-19. Our objective was to measure patient antibody levels before and after transfusion with COVID-19 Convalescent Plasma (CCP) and compare the antibody levels following transfusion of CCP from vaccinated and nonvaccinated donors. Plasma samples before and after transfusion were obtained from 25 recipients of CCP and COVID-19 antibody levels measured. Factors that effect changes in antibody levels were examined. In the 21 patients who received CCP from nonvaccinated donors, modest increases in antibody levels were observed. Patients who received two units were more likely to seroconvert than those receiving just one unit. The strongest predictor of changes in patient antibody level was the CCP dose, calculated by the unit volume multiplied by the donor antibody level. Using patient plasma volume and donor antibody level, the post-transfusion antibody level could be predicted with reasonable accuracy(R2> 0.90). In contrast, the 4 patients who received CCP from vaccinated donors all had dramatic increases in antibody levels following transfusion of a single unit. In this subset of recipients, antibody levels observed after transfusion of CCP were comparable to those seen in donors who had fully recovered from COVID-19. If available, CCP from vaccinated donors with very high antibody levels should be used. One unit of CCP from vaccinated donors increases patient antibody levels much more than 1 or 2 units of CCP from unvaccinated donors.

Exponential increase in neutralizing and spike specific antibodies following vaccination of COVID-19 convalescent plasma donors.

BACKGROUND: With the recent approval of COVID-19 vaccines, recovered COVID-19 subjects who are vaccinated may be ideal candidates to donate COVID-19 convalescent plasma (CCP). CASE SERIES: Eleven recovered COVID-19 patients were screened to donate CCP. All had molecularly confirmed COVID-19, and all but one were antibody positive by chemiluminescence immunoassay (DiaSorin) prior to vaccination. All were tested again for antibodies 11-21 days after they were vaccinated (Pfizer/Moderna). All showed dramatic increases (~50-fold) in spike-specific antibody levels and had at least a 20-fold increase in the IC50 neutralizing antibody titer based on plaque reduction neutralization testing (PRNT). The spike-specific antibody levels following vaccination were significantly higher than those seen in any non-vaccinated COVID-19 subjects tested to date at our facility. CONCLUSION: Spike-specific and neutralizing antibodies demonstrated dramatic increases following a single vaccination after COVID-19 infection, which significantly exceeded values seen with COVID-19 infection alone. Recovered COVID-19 subjects who are vaccinated may make ideal candidates for CCP donation.

COVID-19 treatments and pathogenesis including anosmia in K18-hACE2 mice.

The ongoing coronavirus disease 2019 (COVID-19) pandemic is associated with substantial morbidity and mortality. Although much has been learned in the first few months of the pandemic, many features of COVID-19 pathogenesis remain to be determined. For example, anosmia is a common presentation, and many patients with anosmia show no or only minor respiratory symptoms1. Studies in animals infected experimentally with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of COVID-19, provide opportunities to study aspects of the disease that are not easily investigated in human patients. Although the severity of COVID-19 ranges from asymptomatic to lethal2, most experimental infections provide insights into mild disease3. Here, using K18-hACE2 transgenic mice that were originally developed for SARS studies4, we show that infection with SARS-CoV-2 causes severe disease in the lung and, in some mice, the brain. Evidence of thrombosis and vasculitis was detected in mice with severe pneumonia. Furthermore, we show that infusion of convalescent plasma from a recovered patient with COVID-19 protected against lethal disease. Mice developed anosmia at early time points after infection. Notably, although pre-treatment with convalescent plasma prevented most signs of clinical disease, it did not prevent anosmia. Thus, K18-hACE2 mice provide a useful model for studying the pathological basis of both mild and lethal COVID-19 and for assessing therapeutic interventions.

K18-hACE2 Mice for Studies of COVID-19 Treatments and Pathogenesis Including Anosmia.

The ongoing COVID-19 pandemic is associated with substantial morbidity and mortality. While much has been learned in the first months of the pandemic, many features of COVID-19 pathogenesis remain to be determined. For example, anosmia is a common presentation and many patients with this finding show no or only minor respiratory signs. Studies in animals experimentally infected with SARS-CoV-2, the cause of COVID-19, provide opportunities to study aspects of the disease not easily investigated in human patients. COVID-19 severity ranges from asymptomatic to lethal. Most experimental infections provide insights into mild disease. Here, using K18-hACE2 mice that we originally developed for SARS studies, we show that infection with SARS-CoV-2 causes severe disease in the lung, and in some mice, the brain. Evidence of thrombosis and vasculitis was detected in mice with severe pneumonia. Further, we show that infusion of convalescent plasma (CP) from a recovered COVID-19 patient provided protection against lethal disease. Mice developed anosmia at early times after infection. Notably, while treatment with CP prevented significant clinical disease, it did not prevent anosmia. Thus K18-hACE2 mice provide a useful model for studying the pathological underpinnings of both mild and lethal COVID-19 and for assessing therapeutic interventions.

False positive testing for sickle hemoglobin in a blood donor with mild erythrocytosis and hemoglobin Geldrop St. Anna.

Solubility testing for sickle hemoglobin is commonly performed to identify blood suitable for patients with sickle cell disease. A 32-year-old Caucasian male blood donor's unit screened positive for sickle hemoglobin via solublity testing (Streck). As the donor was considered low risk for being positive for hemoglobin S (HbS), he self-referred to hematology for further evaluation. Testing with hemoglobin electrophoresis revealed the patient to be negative for HbS; however, 42 % fetal hemoglobin (HbF) was noted. Since this was higher than typically seen in hereditary persistence of HbF, deoxyribonucleic acid (DNA) sequencing of hemoglobin (Hb) was ordered through a referral laboratory. Hb gene sequencing revealed the patient to be heterozygous for Hb Geldrop St. Anna, a rare Hb variant. This variant has previously been shown to migrate in the HbF region with alkaline electrophoresis. The workup demonstrated that the oxygen dissociation curve was left-shifted consistent with slightly increased oxygen affinity of this variant. The patient's hematocrits (Hct) from his past donations were 53 % and 54 % about two years apart and his Hct at his hematology evaluation was 53 %. This report describes the first case of Hb Geldrop St. Anna in the United States and was associated with a false positive HbS screen. This Hb variant is considered to be benign and has an increased oxygen affinity that is associated with mild erythrocytosis. The donor was allowed to continue donating blood products.

ABO-incompatible platelets are associated with increased transfusion reaction rates.

BACKGROUND: ABO compatibility can affect platelet transfusion safety and efficacy, and ABO-incompatible (ABOi) platelets likely increases the risks of transfusion reactions though the magnitude of this risk is unclear. STUDY DESIGN AND METHODS: Data collected on all platelet transfusions administered over 36+ months were classified based on patient and product ABO blood group type and merged with a data set that included all transfusion reactions reported during that period. The transfusion reaction rates among various subsets was calculated. RESULTS: In patients greater than 1 year of age, the transfusion reaction rate in the ABO-compatible (ABO-identical) platelet group was 1.0%, while the ABOi platelet group had an elevated reaction rate of 1.7%. The increased reaction rate for ABOi platelets held true even if the analysis were limited to Centers for Disease Control and Prevention/National Healthcare Safety Network qualifying reactions or just allergic or febrile nonhemolytic reactions. The increased reaction rate with ABOi platelets was independent of unit age. Surprisingly, major-incompatible transfusions (A/B antigen incompatible) had the highest rate of reactions, at 2.0%. During the study period, three acute hemolytic reactions were reported out of 2522 plasma-incompatible platelet transfusions (0.12%). CONCLUSIONS: Our results find that compatible platelet transfusions have the lowest rate of transfusion reactions. While hemolytic reactions were observed with plasma-incompatible transfusions, the rate was low. Transfusion of ABO antigen-incompatible platelets had the highest rate of transfusion reactions and resulted in a transfusion reaction rate 1.5 to 2 times that of ABO compatible transfusions.

Predicting changes in hemoglobin S after simple transfusion using complete blood counts.

BACKGROUND: Hemoglobin S percentages are used in the management of patients who have sickle cell disease. However, hemoglobin S measurements often are not routinely or rapidly performed. Rapid and accurate methods to estimate hemoglobin S levels after simple transfusion may improve the care of patients with sickle cell disease. STUDY DESIGN AND METHODS: A comprehensive review of the electronic medical record identified 24 stable patients with sickle cell disease who received simple red blood cell transfusions and had hemoglobin S measurements before and after the transfusion that were less than 72 hours apart. Examination of these patients identified 62 separate transfusions that met our criteria. Three simple equations that utilized complete blood count values and readily available information from the medical record were used to predict the post-transfusion hemoglobin S level after transfusion (Equation 1: predicted post-transfusion hemoglobin = pre-transfusion hemoglobin S × [pre-transfusion hemoglobin/post-transfusion hemoglobin]; Equation 2: predicted post-transfusion hemoglobin S = pre-transfusion hemoglobin S × [pre-transfusion hematocrit/post-transfusion hematocrit]; and Equation 3: predicted post-transfusion hemoglobin S = pre-transfusion hemoglobin S × total pre-transfusion hemoglobin/[total pre-transfusion hemoglobin + (red blood cell volume × 20)]). RESULTS: The predicted hemoglobin S values for all three equations showed a highly significant correlation with the measured post-hemoglobin S value. The coefficient of determination (R2 ) for Equations 1, 2, and 3 was 0.95, 0.92, and 0.97, respectively. Predicting the post-transfusion hemoglobin S value using estimates of the patient's total hemoglobin and the transfused hemoglobin (Equation 3) was the most precise. CONCLUSION: Reductions in hemoglobin S values in patients with sickle cell disease who receive simple red blood cell transfusions can be reliably predicted using complete blood cell measurements and simple arithmetic equations.

Factors influencing platelet clumping during peripheral blood hematopoietic stem cell collection.

BACKGROUND: Platelet clumping is a common occurrence during peripheral blood hematopoietic stem cell (HSC) collection using the Spectra Optia mononuclear cell (MNC) protocol. If clumping persists, it may prevent continuation of the collection and interfere with proper MNC separation. This study is the first to report the incidence of clumping, identify precollection factors associated with platelet clumping, and describe the degree to which platelet clumping interferes with HSC product yield. STUDY DESIGN AND METHODS: In total, 258 HSC collections performed on 116 patients using the Optia MNC protocol were reviewed. Collections utilized heparin in anticoagulant citrate dextrose to facilitate large-volume leukapheresis. Linear and logistic regression models were utilized to determine which precollection factors were predictive of platelet clumping and whether clumping was associated with product yield or collection efficiency. RESULTS: Platelet clumping was observed in 63% of collections. Multivariable analysis revealed that a lower white blood cell count was an independent predictor of clumping occurrence. Chemotherapy mobilization and a lower peripheral blood CD34+ cell count were predictors of the degree of clumping. Procedures with clumping had higher collection efficiency but lower blood volume processed on average, resulting in no difference in collection yields. Citrate toxicity did not correlate with clumping. CONCLUSION: Although platelet clumping is a common technical problem seen during HSC collection, the total CD34+ cell-collection yields were not affected by clumping. WBC count, mobilization approach, and peripheral blood CD34+ cell count can help predict clumping and potentially drive interventions to proactively manage clumping.

Obesity-associated NLRC4 inflammasome activation drives breast cancer progression.

Obesity is associated with an increased risk of developing breast cancer and is also associated with worse clinical prognosis. The mechanistic link between obesity and breast cancer progression remains unclear, and there has been no development of specific treatments to improve the outcome of obese cancer patients. Here we show that obesity-associated NLRC4 inflammasome activation/ interleukin (IL)-1 signalling promotes breast cancer progression. The tumour microenvironment in the context of obesity induces an increase in tumour-infiltrating myeloid cells with an activated NLRC4 inflammasome that in turn activates IL-1ß, which drives disease progression through adipocyte-mediated vascular endothelial growth factor A (VEGFA) expression and angiogenesis. Further studies show that treatment of mice with metformin inhibits obesity-associated tumour progression associated with a marked decrease in angiogenesis. This report provides a causal mechanism by which obesity promotes breast cancer progression and lays out a foundation to block NLRC4 inflammasome activation or IL-1ß signalling transduction that may be useful for the treatment of obese cancer patients.

2-Deoxy-d-glucose Suppresses the In Vivo Antitumor Efficacy of Erlotinib in Head and Neck Squamous Cell Carcinoma Cells.

Poor tumor response to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) is a significant challenge for effective treatment of head and neck squamous cell carcinoma (HNSCC). Therefore, strategies that may increase tumor response to EGFR TKIs are warranted in order to improve HNSCC patient treatment and overall survival. HNSCC tumors are highly glycolytic, and increased EGFR signaling has been found to promote glucose metabolism through various mechanisms. We have previously shown that inhibition of glycolysis with 2-deoxy-d-glucose (2DG) significantly enhanced the antitumor effects of cisplatin and radiation, which are commonly used to treat HNSCC. The goal of the current studies is to determine if 2DG will enhance the antitumor activity of the EGFR TKI erlotinib in HNSCC. Erlotinib transiently suppressed glucose consumption accompanied by alterations in pyruvate kinase M2 (PKM2) expression. 2DG enhanced the cytotoxic effect of erlotinib in vitro but reversed the antitumor effect of erlotinib in vivo. 2DG altered the N-glycosylation status of EGFR and induced the endoplasmic reticulum (ER) stress markers CHOP and BiP in vitro. Additionally, the effects of 2DG + erlotinib on cytotoxicity and ER stress in vitro were reversed by mannose but not glucose or antioxidant enzymes. Lastly, the protective effect of 2DG on erlotinib-induced cytotoxicity in vivo was reversed by chloroquine. Altogether, 2DG suppressed the antitumor efficacy of erlotinib in a HNSCC xenograft mouse model, which may be due to increased cytoprotective autophagy mediated by ER stress activation.

The heritability of hemolysis in stored human red blood cells.

BACKGROUND: The transfusion of red blood cells (RBCs) with maximum therapeutic efficacy is a major goal in transfusion medicine. One of the criteria used in determining stored RBC quality is end-of-storage hemolysis. Between donors, a wide range of hemolysis is observed under identical storage conditions. Here, a potential mechanism for this wide range is investigated. We hypothesize that the magnitude of hemolysis is a heritable trait. Also, we investigated correlations between hemolysis and RBC metabolites; this will establish pathways influencing hemolysis as future targets for genetic analysis. STUDY DESIGN AND METHODS: Units of RBCs from identical and nonidentical twins were collected and stored under standard conditions for 56 days. Hemolysis, adenosine triphosphate (ATP), and total glutathione (tGSH) were measured throughout storage. Nontargeted metabolic analyses were performed on RBCs that had been stored for 28 days. Heritability was determined by comparing values between identical and nonidentical twins. RESULTS: Hemolysis was found to be heritable (mean > 45%) throughout the storage period. Potential correlations were observed between hemolysis and metabolites from the purine metabolism, lysolipid, and glycolysis pathways. These also exhibited heritability (>20%). No correlation was found with ATP or tGSH. CONCLUSION: The susceptibility of RBCs to lysis during storage is partly determined by inheritance. We have also uncovered several pathways that are candidate targets for future genomewide association studies. These findings will aid in the design of better storage solutions and the development of donor screening tools that minimize hemolysis during storage.

Visual and functional demonstration of growing Bax-induced pores in mitochondrial outer membranes.

Bax induces mitochondrial outer membrane permeabilization (MOMP), a critical step in apoptosis in which proteins are released into the cytoplasm. To resolve aspects of the mechanism, we used cryo-electron microscopy (cryo-EM) to visualize Bax-induced pores in purified mitochondrial outer membranes (MOMs). We observed solitary pores that exhibited negative curvature at their edges. Over time, the pores grew to ∼ 100-160 nm in diameter after 60-90 min, with some pores measuring more than 300 nm. We confirmed these results using flow cytometry, which we used to monitor the release of fluorescent dextrans from isolated MOM vesicles. The dextran molecules were released gradually, in a manner constrained by pore size. However, the release rates were consistent over a range of dextran sizes (10-500 kDa). We concluded that the pores were not static but widened dramatically to release molecules of different sizes. Taken together, the data from cryo-EM and flow cytometry argue that Bax promotes MOMP by inducing the formation of large, growing pores through a mechanism involving membrane-curvature stress.

Pathology Milestones: Assessing Clinical Competency by Committee.

All Accreditation Council for Graduate Medical Education accredited pathology residency training programs are now required to evaluate residents using the new Pathology Milestones assessment tool. Similar to implementation of the 6 Accreditation Council for Graduate Medical Education competencies a decade ago, there have been challenges in implementation of the new milestones for many residency programs. The pathology department at the University of Iowa has implemented a process that divides the labor of the task in rating residents while also maintaining consistency in the process. The process is described in detail, and some initial trends in milestone evaluation are described and discussed. Our experience indicates that thoughtful implementation of the Pathology Milestones can provide programs with valuable information that can inform curricular changes.

The heritability of metabolite concentrations in stored human red blood cells.

BACKGROUND: The degeneration of red blood cells (RBCs) during storage is a major issue in transfusion medicine. Family studies in the 1960s established the heritability of the RBC storage lesion based on poststorage adenosine triphosphate (ATP) concentrations. However, this critical discovery has not been further explored. In a classic twin study we confirmed the heritability of poststorage ATP concentrations and established the heritability of many other RBC metabolites. STUDY DESIGN AND METHODS: ATP concentrations and metabolomic profiles were analyzed in RBC samples from 18 twin pairs. On samples stored for 28 days, the heritability of poststorage ATP concentrations were 64 and 53% in CP2D- and AS-3-stored RBCs, respectively. RESULTS: Metabolomic analyses identified 87 metabolites with an estimated heritability of 20% or greater. Thirty-six metabolites were significantly correlated with ATP concentrations (p ≤ 0.05) and 16 correlated with borderline significance (0.05 ≤ p ≤ 0.10). Of the 52 metabolites that correlated significantly with ATP, 24 demonstrated 20% or more heritability. Pathways represented by heritable metabolites included glycolysis, membrane remodeling, redox homeostasis, and synthetic and degradation pathways. CONCLUSION: We conclude that many RBC metabolite concentrations are genetically influenced during storage. Future studies of key metabolic pathways and genetic modifiers of RBC storage could lead to major advances in RBC storage and transfusion therapy.

Low-dose radiation-induced enhancement of thymic lymphomagenesis in Lck-Bax mice is dependent on LET and gender.

The hypothesis that mitochondrial dysfunction and increased superoxide levels in thymocytes over expressing Bax (Lck-Bax1 and Lck-Bax38&1) contributes to lymphomagenesis after low-dose radiation was tested. Lck-Bax1 single-transgenic and Lck-Bax38&1 double-transgenic mice were exposed to single whole-body doses of 10 or 100 cGy of (137)Cs or iron ions (1,000 MeV/n, 150 keV/µm) or silicon ions (300 MeV/n, 67 keV/µm). A 10 cGy dose of (137)Cs significantly increased the incidence and onset of thymic lymphomas in female Lck-Bax1 mice. In Lck-Bax38&1 mice, a 100 cGy dose of high-LET iron ions caused a significant dose dependent acceleration of lymphomagenesis in both males and females that was not seen with silicon ions. To determine the contribution of mitochondrial oxidative metabolism, Lck-Bax38&1 over expressing mice were crossed with knockouts of the mitochondrial protein deacetylase, Sirtuin 3 (Sirt3), which regulates superoxide metabolism. Sirt3(-/-)/Lck-Bax38&1 mice demonstrated significant increases in thymocyte superoxide levels and acceleration of lymphomagenesis (P < 0.001). These results show that lymphomagenesis in Bax over expressing animals is enhanced by radiation exposure in both an LET and gender dependent fashion. These findings support the hypothesis that mitochondrial dysfunction leads to increased superoxide levels and accelerates lymphomagenesis in Lck-Bax transgenic mice.