Aberrant T-cell exhaustion in severe combined immunodeficiency survivors with poor T-cell reconstitution after transplantation

BACKGROUND

Severe combined immunodeficiency (SCID) comprises rare inherited disorders of immunity that require definitive treatment through hematopoietic cell transplantation (HCT) or gene therapy for survival. Despite successes of allogeneic HCT, many SCID patients experience incomplete immune reconstitution, persistent T-cell lymphopenia, and poor long-term outcomes.

OBJECTIVE

We hypothesized that CD4+ T-cell lymphopenia could be associated with a state of T-cell exhaustion in previously transplanted SCID patients.

METHODS

We analyzed markers of exhaustion in blood samples from 61 SCID patients at a median of 10.4 years after HCT.

RESULTS

Compared to post-HCT SCID patients with normal CD4+ T-cell counts, those with poor T-cell reconstitution showed lower frequency of naive CD45RA+/CCR7+ T cells, recent thymic emigrants, and TCR excision circles. They also had a restricted TCR repertoire, increased expression of inhibitory receptors (PD-1, 2B4, CD160, BTLA, CTLA-4), and increased activation markers (HLA-DR, perforin) on their total and naive CD8+ T cells, suggesting T-cell exhaustion and aberrant activation, respectively. The exhaustion score of CD8+ T cells was inversely correlated with CD4+ T-cell count, recent thymic emigrants, TCR excision circles, and TCR diversity. Exhaustion scores were higher among recipients of unconditioned HCT, especially when further in time from HCT. Patients with fewer CD4+ T cells showed a transcriptional signature of exhaustion.

CONCLUSIONS

Recipients of unconditioned HCT for SCID may develop late post-HCT T-cell exhaustion as a result of diminished production of T-lineage cells. Elevated expression of inhibitory receptors on their T cells may be a biomarker of poor long-term T-cell reconstitution.

Abstract 509: Conversion of cellular kinetic data for chimeric antigen receptor T-cell therapy (CAR-T) into interpretable units

Objectives: Cellular kinetic (CK) measurement of CAR-T cell expansion in-vivo by quantitative polymerase chain reaction (qPCR) has been measured in units of transgene copy number/μg of DNA. We propose a formula to convert the qPCR, and flow cytometry (FC) measurements to interpretable numbers of CAR T-cells/μL blood. Further, after the conversion CK parameters were correlated with efficacy/safety endpoints.

Methods: CK data measured using both qPCR and FC assays were utilized from the ELIANA trial in pediatric and young adult patients with relapsed and refractory acute lymphoblastic leukemia (pALL). qPCR measures the presence of CAR transgene in cells with units of copies of CAR-DNA/μg of genomic DNA, while FC quantifies surface expression of CAR T-cells as the % of either T-cells or white blood cells (WBCs) that express CAR. Neither measurement accounts for the typical significant increase in WBCs following CAR-T infusion. We propose equations for converting FC (Eq1) or qPCR (Eq2) into concentration of CAR-cells/μL of blood, using WBC counts from the complete blood count. For qPCR, the equation relies on 3 additional parameters M·F/N: amount of DNA/WBC (M), average number of copies of CAR-DNA/CTL019 cell (N), and fraction of cells with CAR-DNA that express CAR receptor (F). To estimate M·F/N, we performed regression of the cells/μL estimate from FC vs qPCR. We fit the CK model [1] to obtain the model parameters. Finally, CK parameters with safety/efficacy were correlated to compare the converted estimates to the native CK units. • (CD3+CAR+cells)/μl blood= WBC/μl blood × (CD3+CAR+cells)/WBC (1) • (CAR+cells)/μl blood=WBC/μl blood × CAR DNA copies/μg DNA × M μg DNA/WBC × 1 CAR cell/N CAR DNA copies×F (2)

Results: There was high correlation between FC and qPCR estimates of CAR-T in the blood of pALL patients (r2=0.775). The M·F/N value derived based on FC and qPCR results was estimated as 2.68e-6μg DNA/CAR copies. Assuming M=6.6e-6μg DNA/WBC [2] and F=1, this predicts that N=2.46 CAR copies/CAR-T cell. There was also high correlation between the copies/μg and cells/μL estimates using qPCR (r2=0.752). The relationship between CK parameters and safety/efficacy endpoints was not improved when cells/μL was used. This can be attributed to high correlation between these metrics. Using the CK model, 11x greater fold expansion was predicted using the cells/μL estimate compared to copies/μg. This is because cells/μL estimate accounts the expansion of both CAR-T and WBC numbers following lymphodepletion.

Conclusions: The conversion of CK into cells/μL allows for a physiological interpretation of CK data with a high correlation between the cells/μL and copies/μg indicating either metric can be used to predict safety/efficacy.

References: 1 Stein M et al. CPT: Pharmacometrics & Systems Pharmacology(2019) 2 Gillooly JF et al. Cold Spring Harbor Perspectives in Biology vol.7,7 a019091(2015)

Citation Format: Anwesha Chaudhury, Andrew Stein, Stephan Grupp, John Levine, Michael Pulsipher, G Doug Myers, Edward Waldron, Xu Zhu, Fraser McBlane, Rakesh Awasthi, Edmund K. Waller. Conversion of cellular kinetic data for chimeric antigen receptor T-cell therapy (CAR-T) into interpretable units [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 509.

Hematopoietic stem cell transplantation in patients with gain-of-function signal transducer and activator of transcription 1 mutations

BACKGROUND

Gain-of-function (GOF) mutations in signal transducer and activator of transcription 1 (STAT1) cause susceptibility to a range of infections, autoimmunity, immune dysregulation, and combined immunodeficiency. Disease manifestations can be mild or severe and life-threatening. Hematopoietic stem cell transplantation (HSCT) has been used in some patients with more severe symptoms to treat and cure the disorder. However, the outcome of HSCT for this disorder is not well established.

OBJECTIVE

We sought to aggregate the worldwide experience of HSCT in patients with GOF-STAT1 mutations and to assess outcomes, including donor engraftment, overall survival, graft-versus-host disease, and transplant-related complications.

METHODS

Data were collected from an international cohort of 15 patients with GOF-STAT1 mutations who had undergone HSCT using a variety of conditioning regimens and donor sources. Retrospective data collection allowed the outcome of transplantation to be assessed. In vitro functional testing was performed to confirm that each of the identified STAT1 variants was in fact a GOF mutation.

RESULTS

Primary donor engraftment in this cohort of 15 patients with GOF-STAT1 mutations was 74%, and overall survival was only 40%. Secondary graft failure was common (50%), and posttransplantation event-free survival was poor (10% by 100 days). A subset of patients had hemophagocytic lymphohistiocytosis before transplant, contributing to their poor outcomes.

CONCLUSION

Our data indicate that HSCT for patients with GOF-STAT1 mutations is curative but has significant risk of secondary graft failure and death.

Incidence, risk factors, and pathogenesis of second malignancies in patients with non-Hodgkin lymphoma

Most Non-Hodgkin's Lymphoma patients will survive their diagnosis. High dose chemotherapy and autologous stem cell transplantation, and radiation therapy have all been implicated as risk factors to secondary cancer development. Herein, we will review the molecular biology, examine the epidemiologic findings, discuss the impact of both chemotherapy and radiotherapy, and focus on the special populations of pediatrics and high dose chemotherapy and autologous stem cell transplantation with regard to secondary cancer development.

Hematopoietic cell transplantation-comorbidity index and Karnofsky performance status are independent predictors of morbidity and mortality after allogeneic nonmyeloablative hematopoietic cell transplantation

BACKGROUND

Elderly and medically infirm cancer patients are increasingly offered allogeneic nonmyeloablative hematopoietic cell transplantation (HCT). A better understanding of the impact of health status on HCT outcomes is warranted. Herein, a recently developed HCT-specific comorbidity index (HCT-CI) was compared with a widely acceptable measure of health status, the Karnofsky performance status (KPS).

METHODS

The outcomes of 341 patients were evaluated, conditioned for either related or unrelated HCT by 2-gray (Gy) total body irradiation given alone or combined with fludarabine at a dose of 90 mg/m(2). Comorbidities were assessed retrospectively by the HCT-CI. Performance status before and toxicities after HCT were graded prospectively using the KPS and National Cancer Institute Common Toxicity criteria, respectively.

RESULTS

Weak Spearman rank correlations were noted between HCT-CI and KPS and between the 2 measures and age, number of prior chemotherapy regimens, and intervals between diagnosis and HCT (all r < 0.20). High-risk diseases correlated significantly with higher mean HCT-CI scores (P = .009) but not low KPS (P = .37). In multivariate models, the HCT-CI had significantly greater independent predictive power for toxicities (P = .004), nonrelapse mortality (P = .0002), and overall mortality (P = .0002) compared with the KPS (P = .05, .13, and .05, respectively). Using consolidated HCT-CI and KPS scores, patients were stratified into 4 risk groups with 2-year survivals of 68%, 58%, 41%, and 32%, respectively.

CONCLUSIONS

HCT-CI and KPS should be assessed simultaneously before HCT. The use of both tools combined likely refines risk-stratification for HCT outcomes. Novel guidelines for assessment of performance status among HCT patients are warranted.

Unrelated donor granulocyte colony-stimulating factor-mobilized peripheral blood mononuclear cell transplantation after nonmyeloablative conditioning: the effect of postgrafting mycophenolate mofetil dosing

We previously reported results in 71 patients with advanced hematologic malignancies given HLA-matched unrelated granulocyte colony-stimulating factor-mobilized peripheral blood mononuclear cell (G-PBMC) grafts after fludarabine 90 mg/m(2), 2 Gy of total body irradiation, and postgrafting mycophenolate mofetil (MMF) 15 mg/kg twice daily and cyclosporine 6.25 mg/kg twice daily orally. Graft rejection was 15%; the cumulative probability of acute graft-versus-host disease (GVHD) was 52%. According to MMF pharmacokinetic studies, which showed a short half-life of its active metabolite, mycophenolic acid, we increased MMF dosing from 15 mg/kg twice daily to 15 mg/kg 3 times daily to increase immunosuppression and reduce the incidence of both graft rejection and acute GVHD. Among 103 patients so treated, graft rejection occurred in 5%, whereas acute GVHD remained at 53%. Outcomes were compared with results of previous G-PBMC recipients given MMF twice daily. Infection rates were slightly higher with MMF 3 times daily than with MMF twice daily. Nevertheless, 2-year nonrelapse mortality and overall and progression-free survivals were similar for MMF 3-times-daily and twice-daily patients (19%, 58%, and 49% versus 20%, 48%, and 37%, respectively). Nonmyeloablative conditioning with postgrafting cyclosporine and MMF given 3 times daily allowed 95% durable engraftment of unrelated donor G-PBMC grafts.

High-risk neuroblastoma treated with tandem autologous peripheral-blood stem cell-supported transplantation: long-term survival update

PURPOSE

To provide an update on long-term survival of patients with high-risk neuroblastoma treated with tandem cycles of myeloablative therapy and peripheral-blood stem-cell rescue (PBSCR).

PATIENTS AND METHODS

Ninety-seven patients with high-risk neuroblastoma were treated between 1994 and 2002. Patients underwent induction therapy with five cycles of standard agents, resection of the primary tumor and local radiation, and two consecutive courses of myeloablative therapy (including total-body irradiation) with PBSCR.

RESULTS

Fifty-one patients have experienced relapse or died. Median follow-up time among the 46 patients who remain alive without progression is 5.6 years (range, 15.1 months to 9.9 years). Progression-free survival (PFS) rate at 5 years from diagnosis was 47% (95% CI, 36% to 56%), and PFS rate at 7 years was 45% (95% CI, 34% to 55%). Overall survival rate was 60% (95% CI, 48% to 69%) and 53% (95% CI, 40% to 64%) at 5 and 7 years, respectively. The 5- and 7- year PFS rates from time of first transplantation for 82 patients who completed both transplants were 54% (95% CI, 42% to 64%) and 52% (95% CI, 40% to 63%), respectively. Five patients died from treatment-related toxicity after tandem transplantation. Relapse occurred in 37 (42%) of 89 patients, mainly within 3 years of transplantation and primarily in diffuse osseous sites. No primary CNS relapse or secondary leukemia was seen. One patient developed synovial cell sarcoma 8 years after therapy.

CONCLUSION

High-dose therapy with tandem autologous stem-cell rescue is effective for treating high-risk neuroblastoma, with encouraging long-term survival. CNS relapse and secondary malignancies are rare after this therapy.

Hematopoietic cell transplantation after nonmyeloablative conditioning for advanced chronic lymphocytic leukemia

PURPOSE

Patients with chemotherapy-refractory chronic lymphocytic leukemia (CLL) have a short life expectancy. The aim of this study was to analyze the outcome of patients with advanced CLL when treated with nonmyeloablative conditioning and hematopoietic cell transplantation (HCT).

PATIENTS AND METHODS

Sixty-four patients diagnosed with advanced CLL were treated with nonmyeloablative conditioning (2 Gy total-body irradiation with [n = 53] or without [n = 11] fludarabine) and HCT from related (n = 44) or unrelated (n = 20) donors. An adapted form of the Charlson comorbidity index was used to assess pretransplantation comorbidities.

RESULTS

Sixty-one of 64 patients had sustained engraftment, whereas three patients rejected their grafts. The incidences of grades 2, 3, and 4 acute and chronic graft-versus-host disease were 39%, 14%, 2%, and 50%, respectively. Three patients who underwent transplantation in complete remission (CR) remained in CR. The overall response rate among 61 patients with measurable disease was 67% (50% CR), whereas 5% had stable disease. All patients with morphologic CR who were tested by polymerase chain reaction (n = 11) achieved negative molecular results, and one of these patients subsequently experienced disease relapse. The 2-year incidence of relapse/progression was 26%, whereas the 2-year relapse and nonrelapse mortalities were 18% and 22%, respectively. Two-year rates of overall and disease-free survivals were 60% and 52%, respectively. Unrelated HCT resulted in higher CR and lower relapse rates than related HCT, suggesting more effective graft-versus-leukemia activity.

CONCLUSION

CLL is susceptible to graft-versus-leukemia effects, and allogeneic HCT after nonmyeloablative conditioning might prolong median survival for patients with advanced CLL.

Allogeneic hematopoietic cell transplantation after fludarabine and 2 Gy total body irradiation for relapsed and refractory mantle cell lymphoma

We carried out HLA-matched related (n = 16) and unrelated (n = 17) hematopoietic cell transplantation (HCT) in 33 patients with relapsed and refractory mantle cell lymphoma after nonmyeloablative conditioning with fludarabine and 2 Gy total body irradiation. Postgrafting immunosuppression consisted of cyclosporine and mycophenolate mofetil. Fourteen patients had failed high-dose autologous HCT. Of the 33 patients studied, 31 had stable engraftment, whereas 2 patients experienced nonfatal graft rejections. The incidences of acute grades II, III, and IV, and chronic graft-versus-host disease (GVHD) were 27%, 17%, 13%, and 64%, respectively. The overall response rate in the 20 patients with measurable disease at the time of HCT was 85% (n = 17; 75% complete remissions [CR] and 10% partial remissions [PR]), whereas 3 patients had progressive disease. Only one of the 17 patients who responded and none of the 13 who received transplants in CR had disease relapse with a median follow-up of 24.6 months. Relapse and nonrelapse mortalities were 9% and 24%, respectively, at 2 years. The Kaplan-Meier probabilities of overall and disease-free survivals at 2 years were 65% and 60%, respectively. Allogeneic HCT after nonmyeloablative conditioning is a promising salvage strategy for patients with relapsed and refractory mantle cell lymphoma. The high response and low relapse rates with this approach suggest that mantle cell lymphoma is susceptible to graft-versus-tumor responses.

HLA-matched unrelated donor hematopoietic cell transplantation after nonmyeloablative conditioning for patients with hematologic malignancies

A hematopoietic cell transplantation (HCT) approach was developed for elderly or ill patients with hematologic malignancies that employed nonmyeloablative conditioning to avoid common regimen-related toxicities and relied on graft-versus-tumor effects for control of malignancy. Eighty-nine patients, median age 53 years, were given fludarabine (90 mg/m2) and 2 Gy total body irradiation. Marrow (n = 18) or granulocyte colony-stimulating factor (G-CSF)-stimulated peripheral blood mononuclear cells (G-PBMCs; n = 71) were transplanted from unrelated donors matched for human leukocyte antigen A (HLA-A), -B, -C antigens and -DRB1 and -DQB1 alleles. Postgrafting immunosuppression included mycophenolate mofetil and cyclosporine. Donor T-cell chimerism was higher for G-PBMCs compared with marrow recipients. Durable engraftment was observed in 85% of G-PBMCs and 56% of marrow recipients. Cumulative probabilities of grade II, III, and IV acute graft-versus-host disease (GVHD) were 42%, 8%, and 2%, respectively. Nonrelapse mortality at day 100 and at 1 year was 11% and 16%, respectively. One-year overall survivals and progression-free survivals were 52% and 38%, respectively. G-PBMC recipients had improved survival (57% vs 33%) and progression-free survival (44% vs 17%) compared with marrow recipients. HLA-matched unrelated donor HCT after nonmyeloablative conditioning is feasible in patients ineligible for conventional HCT. G-PBMCs conferred higher donor T-cell chimerism, greater durable engraftment, and better progression-free and overall survivals compared with marrow.

Reduced toxicity and prompt engraftment after minimal conditioning of a patient with Fanconi anemia undergoing hematopoietic stem cell transplantation from an HLA-matched unrelated donor

Given the profound sensitivity of patients with Fanconi anemia to conventional conditioning regimens before hematopoietic stem cell transplantation (HSCT), we developed a minimally toxic regimen consisting of 2 Gy total body irradiation, 90 mg/m2 fludarabine, and postgrafting immunosuppression with cyclosporine and mycophenolate to treat FA patients undergoing HSCT from HLA-matched unrelated donors. With over 10 months follow-up, our first patient has complete and sustained engraftment. Graft-versus-host disease was limited to mild skin and liver and moderate gut manifestations. We conclude that the approach is well tolerated and ideally suited to reduce regimen-related toxicities while achieving sustained engraftment.

Using fluorescence-activated cell sorting followed by fluorescence in situ hybridization to study lineage relationships: the 8;21 translocation is present in neutrophils but not monocytes in a patient with severe congenital neutropenia and a granulocyte colony-stimulating factor-responsive clonal abnormality

Severe congenital neutropenia (Kostmann syndrome) is a disorder that presents in the neonatal period, but predisposes to leukemia later in life. This report describes a 4-y-old female with a history of severe congenital neutropenia, who developed a clonal abnormality associated with the translocation (7;21;8) (q32;q22;q22) (AML-1/ETO). She had circulating peripheral blasts and bone marrow blast counts as high as 64% when she received recombinant granulocyte colony-stimulating factor (rG-CSF). Her marrow blasts decreased to 4-20% when rG-CSF was discontinued. Fluorescence in situ hybridization analysis was performed on bone marrow cell populations sorted by flow cytometry to determine which cell populations had the AML-1/ETO translocation. The translocation was found in mature neutrophils and blasts, but not in monocytes, lymphocytes or stem cells.

CONCLUSION

These findings suggest that the translocation occurred in a neutrophil progenitor, past the point in ontogeny where monocytes and neutrophils separate. The techniques described may be useful in understanding lineage relationships and leukemogenesis in other clonal abnormalities associated with myelodysplasia and leukemia.

One-day ex vivo culture allows effective gene transfer into human nonobese diabetic/severe combined immune-deficient repopulating cells using high-titer vesicular stomatitis virus G protein pseudotyped retrovirus

Retrovirus-mediated gene transfer into long-lived human pluripotent hematopoietic stem cells (HSCs) is a widely sought but elusive goal. A major problem is the quiescent nature of most HSCs, with the perceived requirement for ex vivo prestimulation in cytokines to induce stem cell cycling and allow stable gene integration. However, ex vivo culture may impair stem cell function, and could explain the disappointing clinical results in many current gene transfer trials. To address this possibility, we examined the ex vivo survival of nonobese diabetic/severe combined immune-deficient (NOD/SCID) repopulating cells (SRCs) over 3 days. After 1 day of culture, the SRC number and proliferation declined twofold, and was further reduced by day 3; self-renewal was only detectable in noncultured cells. To determine if the period of ex vivo culture could be shortened, we used a vesicular stomatitis virus G protein (VSV-G) pseudotyped retrovirus vector that was concentrated to high titer. The results showed that gene transfer rates were similar without or with 48 hours prestimulation. Thus, the use of high-titer VSV-G pseudotyped retrovirus may minimize the loss of HSCs during culture, because efficient gene transfer can be obtained without the need for extended ex vivo culture.

Subtyping analysis of Fanconi anemia by immunoblotting and retroviral gene transfer

Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome with at least eight complementation groups (A-H). Two of the FA genes (FAA and FAC) have been cloned, and mutations in these genes account for approximately 80% of FA patients. Subtyping of FA patients is an important first step toward identifying candidates for FA gene therapy. In the current study, we analyzed a reference group of 26 FA patients of known subtype. Most of the patients (18/26) were confirmed as either type A or type C by immunoblot analysis with anti-FAA and anti-FAC antisera. In order to resolve the subtype of the remaining patients, we generated retroviral constructs expressing FAA and FAC for transduction of FA cell lines (pMMP-FAA and pMMP-FAC). The pMMP-FAA construct specifically complemented the abnormal phenotype of cell lines from FA-A patients, while pMMP-FAC complemented FA-C cells. In summary, the combination of immunoblot analysis and retroviral-mediated phenotypic correction of FA cells allows a rapid method of FA subtyping.

The Fanconi anaemia proteins, FAA and FAC, interact to form a nuclear complex

Fanconi anaemia (FA) is an autosomal-recessive disorder characterized by genomic instability, developmental defects, DNA crosslinking agent hypersensitivity and cancer susceptibility. Somatic-cell hybrid studies have revealed five FA complementation groups (A-E; refs 4-6) displaying similar phenotypes, suggesting that FA genes are functionally related. The two cloned FA genes, FAA and FAC, encode proteins that are unrelated to each other or to other proteins in GenBank. In the current study, we demonstrate the FAA and FAC bind each other and form a complex. Protein binding correlates with the functional activity of FAA and FAC, as patient-derived mutant FAC (L554P) fails to bind FAA. Although unbound FAA and FAC localize predominantly to the cytoplasm, the FAA-FAC complex is found in similar abundance in both cytoplasm and nucleus. Our results confirm the interrelatedness of the FA genes in a pathway, suggesting the cooperation of FAA and FAC in a nuclear function.

Selective transgene expression for detection and elimination of contaminating carcinoma cells in hematopoietic stem cell sources

Tumor contamination of bone marrow (BM) and peripheral blood (PB) may affect the outcome of patients receiving high dose chemotherapy with autologous transplantation of hematopoietic stem cell products. In this report, we demonstrate that replication defective adenoviral vectors containing the cytomegalovirus (CMV) or DF3/MUC1 carcinoma-selective promoter can be used to selectively transduce contaminating carcinoma cells. Adenoviral-mediated reporter gene expression in breast cancer cells was five orders of magnitude higher than that found in BM, PB, and CD34+ cells. Our results demonstrate that CD34+ cells have low to undetectable levels of integrins responsible for adenoviral internalization. We show that adenoviral-mediated transduction of a reporter gene can detect one breast cancer cell in 5 x 10(5) BM or PB cells with a vector containing the DF3/MUC1 promoter. We also show that transduction of the HSV-tk gene for selective killing by ganciclovir can be exploited for purging cancer cells from hematopoietic stem cell populations. The selective expression of TK followed by ganciclovir treatment resulted in the elimination of 6-logs of contaminating cancer cells. By contrast, there was little effect on CFU-GM and BFU-E formulation or on long term culture initiating cells. These results indicate that adenoviral vectors with a tumor-selective promoter provide a highly efficient and effective approach for the detection and purging of carcinoma cells in hematopoietic stem cell preparations.

Quantitative aspects of metal ion binding to certain transfer RNA anticodon loop modified nucleosides

Magnesium and manganese ions bind strongly to the unusual transfer RNA anticodon loop nucleotides, N-[(9-beta-D-ribofuranosyl-9H-purin-6-yl)carbamoyl]-L-threonine 5'-monophosphate (pt6A) and uridine-5-oxyacetic acid 5'-monophosphate (pV). Potentiometric measurements have shown that the delta G for metal ion-pt6A complex formation is 2-3-times more exothermic than for AMP. Electron-nuclear longitudinal dipolar relaxation data yielded manganese-ligand atom distances which permit a three-dimensional construct of the complex in which metal is coordinated to the phosphate, carboxylate of the threonine side-chain (with the nucleotide in the anti glycosidic conformation) and N7 of the adenine ring. Similarly, manganese binds strongly to pV, involving phosphate and carboxylate functions. It is possible that a facet of the functional role of these unusual residues is to chelate magnesium ions and in so doing permit optimum anticodon loop conformational stability and stability of tRNA-mRNA-ribosome complexes.