Product Development within the National Institutes of Health Radiation and Nuclear Countermeasures Program

The Radiation and Nuclear Countermeasures Program (RNCP) at the National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH) was established to facilitate the development of medical countermeasures (MCMs) and diagnostic approaches for use in a radiation public health emergency. Approvals for MCMs can be very challenging but are made possible under the United States Food and Drug Administration (FDA) Animal Rule, which is designed to enable licensure of drugs or biologics when clinical efficacy studies are unethical or unfeasible. The NIAID portfolio includes grants, contracts, and inter-agency agreements designed to span all aspects of drug development and encompasses basic research through FDA approval. In addition, NIAID manages an active portfolio of biodosimetry approaches to assess injuries and absorbed radiation levels to guide triage and treatment decisions. NIAID, together with grantees, contractors, and other stakeholders with promising products, works to advance candidate MCMs and biodosimetry tools through an established product development pipeline. In addition to managing grants and contracts, NIAID tests promising candidates in our established preclinical animal models, and the NIAID Program Officers work closely with sponsors as product managers to guide them through the process. In addition, a valuable benefit for stakeholders is working with the NIAID Office of Regulatory Affairs, where NIAID coordinates with the FDA to facilitate interactions between sponsors and the agency. Activities funded by NIAID include basic research (e.g., library screens to discover new products, determine early efficacy, and delineate mechanism of action) and the development of small and large animal models of radiation-induced hematopoietic, gastrointestinal, lung, kidney, and skin injury, radiation combined injury, and radionuclide decorporation. NIAID also sponsors Good Laboratory Practice product safety, pharmacokinetic, pharmacodynamic, and toxicology studies, as well as efficacy and dose-ranging studies to optimize product regimens. For later-stage candidates, NIAID funds large-scale manufacturing and formulation development of products. The program also supports Phase 1 human clinical studies to ensure human safety and to bridge pharmacokinetic, pharmacodynamic, and efficacy data from animals to humans. To date, NIAID has supported >900 animal studies and one clinical study, evaluating >500 new/repurposed radiation MCMs and biodosimetric approaches. NIAID sponsorship led to the approval of three of the four drugs for acute radiation syndrome under the FDA Animal Rule, five Investigational New Drug applications, and 18 additional submissions for Investigational Device Exemptions, while advancing 38 projects to the Biomedical Advanced Research and Development Authority for follow-on research and development.

Radiation-induced multi-organ injury

PURPOSE

Natural history studies have been informative in dissecting radiation injury, isolating its effects, and compartmentalizing injury based on the extent of exposure and the elapsed time post-irradiation. Although radiation injury models are useful for investigating the mechanism of action in isolated subsyndromes and development of medical countermeasures (MCMs), it is clear that ionizing radiation exposure leads to multi-organ injury (MOI).

METHODS

The Radiation and Nuclear Countermeasures Program within the National Institute of Allergy and Infectious Diseases partnered with the Biomedical Advanced Research and Development Authority to convene a virtual two-day meeting titled 'Radiation-Induced Multi-Organ Injury' on June 7-8, 2022. Invited subject matter experts presented their research findings in MOI, including study of mechanisms and possible MCMs to address complex radiation-induced injuries.

RESULTS

This workshop report summarizes key information from each presentation and discussion by the speakers and audience participants.

CONCLUSIONS

Understanding the mechanisms that lead to radiation-induced MOI is critical to advancing candidate MCMs that could mitigate the injury and reduce associated morbidity and mortality. The observation that some of these mechanisms associated with MOI include systemic injuries, such as inflammation and vascular damage, suggests that MCMs that address systemic pathways could be effective against multiple organ systems.

Immune Dysfunction from Radiation Exposure

Exposure to ionizing radiation causes acute damage and loss of bone marrow and peripheral immune cells that can result in high mortality due to reduced resistance to infections and hemorrhage. Besides these acute effects, tissue damage from radiation can trigger inflammatory responses, leading to progressive and chronic tissue damage by radiation-induced loss of immune cell types that are required for resolving tissue injuries. Understanding the mechanisms involved in radiation-induced immune system injury and repair will provide new insights for developing medical countermeasures that help restore immune homeostasis. For these reasons, The Radiation and Nuclear Countermeasures Program (RNCP) and the Basic Immunology Branch (BIB) under the Division of Allergy, Immunology, and Transplantation (DAIT) within the National Institute of Allergy and Infectious Diseases (NIAID) convened a two-day workshop, along with partners from the Biomedical Advanced Research and Development Authority (BARDA), and the Radiation Injury Treatment Network (RITN). This workshop, titled "Immune Dysfunction from Radiation Exposure," was held virtually on September 9-10, 2020; this Commentary provides a high-level overview of what was discussed at the meeting.

Immune Dysfunction from Radiation Exposure

The hematopoietic system is highly sensitive to ionizing radiation. Damage to the immune system may result in opportunistic infections and hemorrhage, which could lead to mortality. Inflammation triggered by tissue damage can also lead to additional local or widespread tissue damage. The immune system is responsible for tissue repair and restoration, which is made more challenging when it is in the process of self-recovery. Because of these challenges, the Radiation and Nuclear Countermeasures Program (RNCP) and the Basic Immunology Branch (BIB) under the Division of Allergy, Immunology, and Transplantation (DAIT) within the National Institute of Allergy and Infectious Diseases (NIAID), along with partners from the Biomedical Advanced Research and Development Authority (BARDA), and the Radiation Injury Treatment Network (RITN) sponsored a two-day meeting titled Immune Dysfunction from Radiation Exposure held on September 9-10, 2020. The intent was to discuss the manifestations and mechanisms of radiation-induced immune dysfunction in people and animals, identify knowledge gaps, and discuss possible treatments to restore immune function and enhance tissue repair after irradiation.

The NIAID/RNCP Biodosimetry Program: An Overview

Established in 2004, the Radiation and Nuclear Countermeasures Program (RNCP), within the National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health has the central mission to advance medical countermeasure mitigators/therapeutics, and biomarkers and technologies to assess, triage, and inform medical management of patients experiencing acute radiation syndrome and/or the delayed effects of acute radiation exposure. The RNCP biodosimetry mission space encompasses: (1) basic research to elucidate novel approaches for rapid and accurate assessment of radiation exposure, (2) studies to support advanced development for US Food and Drug Administration (FDA) clearance of promising triage or treatment devices/approaches, (3) characterization of biomarkers and/or assays to determine degree of tissue or organ dose that can predict outcome of radiation injuries (i.e., organ failure, morbidity, and/or mortality), and (4) outreach efforts to facilitate interactions with researchers developing cutting edge biodosimetry approaches. Thus far, no biodosimetry device has been FDA cleared for use during a radiological/nuclear incident. At NIAID, advancement of radiation biomarkers and biodosimetry approaches is facilitated by a variety of funding mechanisms (grants, contracts, cooperative and interagency agreements, and Small Business Innovation Research awards), with the objective of advancing devices and assays toward clearance, as outlined in the FDA's Radiation Biodosimetry Medical Countermeasure Devices Guidance. The ultimate goal of the RNCP biodosimetry program is to develop and establish accurate and reliable biodosimetry tools that will improve radiation preparedness and ultimately save lives during a radiological or nuclear incident.

Considerations of Medical Preparedness to Assess and Treat Various Populations During a Radiation Public Health Emergency

During a radiological or nuclear public health emergency, given the heterogeneity of civilian populations, it is incumbent on medical response planners to understand and prepare for a potentially high degree of interindividual variability in the biological effects of radiation exposure. A part of advanced planning should include a comprehensive approach, in which the range of possible human responses in relation to the type of radiation expected from an incident has been thoughtfully considered. Although there are several reports addressing the radiation response for special populations (as compared to the standard 18-45-year-old male), the current review surveys published literature to assess the level of consideration given to differences in acute radiation responses in certain sub-groups. The authors attempt to bring clarity to the complex nature of human biology in the context of radiation to facilitate a path forward for radiation medical countermeasure (MCM) development that may be appropriate and effective in special populations. Consequently, the focus is on the medical (as opposed to logistical) aspects of preparedness and response. Populations identified for consideration include obstetric, pediatric, geriatric, males, females, individuals of different race/ethnicity, and people with comorbidities. Relevant animal models, biomarkers of radiation injury, and MCMs are highlighted, in addition to underscoring gaps in knowledge and the need for consistent and early inclusion of these populations in research. The inclusion of special populations in preclinical and clinical studies is essential to address shortcomings and is an important consideration for radiation public health emergency response planning. Pursuing this goal will benefit the population at large by considering those at greatest risk of health consequences after a radiological or nuclear mass casualty incident.

Making connections: the scientific impact and mentoring legacy of Dr. John E. Moulder

PURPOSE

The intent of this mini review is to pay homage to Dr. John E. Moulder's long and successful career in radiation science with the Medical College of Wisconsin. This effort will be done from the perspective of his history of U.S. Government funding for research into the biological pathways involved in radiation-induced normal tissue injuries, especially damage to the kidneys and heart, and pharmacological interventions. In addition, the impact of his steady guidance and leadership in the mentoring of junior scientists, and the development of meaningful collaborations with other researchers will be highlighted.

CONCLUSION

Dr. John E. Moulder's contributions to the field of radiation research, through his strong character and reputation, his consistent and dedicated commitment to his colleagues and students, and his significant scientific advances, have been critical to moving the science forward, and will not be forgotten by those who knew him personally or through publications documenting his important work.

NIH Policies and Regulatory Pathways to U.S. FDA licensure: Strategies to Inform Advancement of Radiation Medical Countermeasures and Biodosimetry Devices

The Radiation and Nuclear Countermeasures Program within the National Institute of Allergy and Infectious Diseases (NIAID), is tasked with the mandate of identifying biodosimetry tests to assess exposure and medical countermeasures (MCMs) to mitigate/treat injuries to individuals exposed to significant doses of ionizing radiation from a radiological/nuclear incident, hosted. To fulfill this mandate, the Radiation and Nuclear Countermeasures Program (RNCP), hosted a workshop in 2018 workshop entitled "Policies and Regulatory Pathways to U.S. FDA licensure: Radiation Countermeasures and Biodosimetry Devices." The purpose of the meeting was to facilitate the advancement of MCMs and biodosimetry devices by assessing the research devices and animal models used in preclinical studies; government policies on reproducibility, rigor and robustness; regulatory considerations for MCMs and biodosimetry devices; and lessons learned from sponsors of early stage MCM or biodosimetry devices. Meeting presentations were followed by a NIAID-led, open discussion among academic investigators, industry researchers and U.S. government representatives.

A Trans-Agency Workshop on the Pathophysiology of Radiation-Induced Lung Injury

As of January 2021, the U.S. Food and Drug Administration has approved four radiation exposure medical countermeasures (MCMs) to treat hematological acute effects, but no MCM is yet approved for radiation-induced lung injury (RILI). MCM approval for RILI and other subsyndromes utilizes the FDA Animal Efficacy Rule (Animal Rule), that requires demonstration of MCM efficacy in animal models with well-characterized pathophysiology, therefore, allowing translation to human use. A good animal model replicates the clinical condition and natural history of the disease, while allowing for studying the mechanism of action of the applied MCM and exhibiting clear benefits in terms of primary and secondary endpoints. However, there is much conversation regarding the advantages and limitations of individual models, and how to properly apply these models to demonstrate MCM efficacy. On March 20, 2019, the Radiation and Nuclear Countermeasures Program (RNCP) within the National Institute of Allergy and Infectious Diseases (NIAID), Food and Drug Administration (FDA), and the Biomedical Advanced Research and Development Authority (BARDA) sponsored a workshop to identify critical research gaps, discuss current clinical practices for different types of pulmonary diseases, and consider available animal models for RILI.

Meeting Report: A Poly-Pharmacy Approach to Mitigate Acute Radiation Syndrome

The National Institute of Allergy and Infectious Diseases, Radiation and Nuclear Countermeasures Program, was tasked by the United States Congress and the U.S. Department of Health and Human Services to identify and fund early-to-mid-stage development of medical countermeasures (MCMs) to treat radiation-induced injuries. In developing MCMs to treat various sub-syndromes (e.g., hematopoietic, gastrointestinal, lung), it is important to investigate whether a poly-pharmacy approach (i.e., drug cocktails) can provide additive benefits to mitigate injuries arising from the acute radiation syndrome (ARS). In addition, potential drug-drug interactions must be examined. For this reason, a workshop was held, which centered on understanding the current state of research investigating poly-pharmacy approaches to treat radiation injuries. The first session set the stage with an introduction to the concept of operations or support available for the response to a nuclear incident, as this is the key to any emergency response, including MCM availability and distribution. The second session followed the natural history of ARS in both humans and animal models to underscore the complexity of ARS and why a poly-pharmacy approach may be necessary. The third session featured talks from investigators conducting current MCM poly-pharmacy research. The meeting closed with a focus on regulatory considerations for the development of poly-pharmacy approaches or combination treatments for ARS.

A Trans-Agency Workshop on the Pathophysiology of Radiation-Induced Lung Injury

Research and development of medical countermeasures (MCMs) for radiation-induced lung injury relies on the availability of animal models with well-characterized pathophysiology, allowing effective bridging to humans. To develop useful animal models, it is important to understand the clinical condition, advantages and limitations of individual models, and how to properly apply these models to demonstrate MCM efficacy. On March 20, 2019, a meeting sponsored by the Radiation and Nuclear Countermeasures Program (RNCP) within the National Institute of Allergy and Infectious Diseases (NIAID) brought together medical, scientific and regulatory communities, including academic and industry subject matter experts, and government stakeholders from the Food and Drug Administration (FDA) and the Biomedical Advanced Research and Development Authority (BARDA), to identify critical research gaps, discuss current clinical practices for various forms of pulmonary damage, and consider available animal models for radiation-induced lung injury.

Acute Radiation Syndrome and the Microbiome: Impact and Review

Study of the human microbiota has been a centuries-long endeavor, but since the inception of the National Institutes of Health (NIH) Human Microbiome Project in 2007, research has greatly expanded, including the space involving radiation injury. As acute radiation syndrome (ARS) is multisystemic, the microbiome niches across all areas of the body may be affected. This review highlights advances in radiation research examining the effect of irradiation on the microbiome and its potential use as a target for medical countermeasures or biodosimetry approaches, or as a medical countermeasure itself. The authors also address animal model considerations for designing studies, and the potential to use the microbiome as a biomarker to assess radiation exposure and predict outcome. Recent research has shown that the microbiome holds enormous potential for mitigation of radiation injury, in the context of both radiotherapy and radiological/nuclear public health emergencies. Gaps still exist, but the field is moving forward with much promise.

Commonalities Between COVID-19 and Radiation Injury

As the multi-systemic components of COVID-19 emerge, parallel etiologies can be drawn between SARS-CoV-2 infection and radiation injuries. While some SARS-CoV-2-infected individuals present as asymptomatic, others exhibit mild symptoms that may include fever, cough, chills, and unusual symptoms like loss of taste and smell and reddening in the extremities (e.g., "COVID toes," suggestive of microvessel damage). Still others alarm healthcare providers with extreme and rapid onset of high-risk indicators of mortality that include acute respiratory distress syndrome (ARDS), multi-organ hypercoagulation, hypoxia and cardiovascular damage. Researchers are quickly refocusing their science to address this enigmatic virus that seems to unveil itself in new ways without discrimination. As investigators begin to identify early markers of disease, identification of common threads with other pathologies may provide some clues. Interestingly, years of research in the field of radiation biology documents the complex multiorgan nature of another disease state that occurs after exposure to high doses of radiation: the acute radiation syndrome (ARS). Inflammation is a key common player in COVID-19 and ARS, and drives the multi-system damage that dramatically alters biological homeostasis. Both conditions initiate a cytokine storm, with similar pro-inflammatory molecules increased and other anti-inflammatory molecules decreased. These changes manifest in a variety of ways, with a demonstrably higher health impact in patients having underlying medical conditions. The potentially dramatic human impact of ARS has guided the science that has identified many biomarkers of radiation exposure, established medical management strategies for ARS, and led to the development of medical countermeasures for use in the event of a radiation public health emergency. These efforts can now be leveraged to help elucidate mechanisms of action of COVID-19 injuries. Furthermore, this intersection between COVID-19 and ARS may point to approaches that could accelerate the discovery of treatments for both.

Study logistics that can impact medical countermeasure efficacy testing in mouse models of radiation injury

PURPOSE

To address confounding issues that have been noted in planning and conducting studies to identify biomarkers of radiation injury, develop animal models to simulate these injuries, and test potential medical countermeasures to mitigate/treat damage caused by radiation exposure.

METHODS

The authors completed an intensive literature search to address several key areas that should be considered before embarking on studies to assess efficacy of medical countermeasure approaches in mouse models of radiation injury. These considerations include: (1) study variables; (2) animal selection criteria; (3) animal husbandry; (4) medical management; and (5) radiation attributes.

RESULTS

It is important to select mouse strains that are capable of responding to the selected radiation exposure (e.g. genetic predispositions might influence radiation sensitivity and proclivity to certain phenotypes of radiation injury), and that also react in a manner similar to humans. Gender, vendor, age, weight, and even seasonal variations are all important factors to consider. In addition, the housing and husbandry of the animals (i.e. feed, environment, handling, time of day of irradiation and animal restraint), as well as the medical management provided (e.g. use of acidified water, antibiotics, routes of administration of drugs, consideration of animal numbers, and euthanasia criteria) should all be addressed. Finally, the radiation exposure itself should be tightly controlled, by ensuring a full understanding and reporting of the radiation source, dose and dose rate, shielding and geometry of exposure, while also providing accurate dosimetry. It is important to understand how all the above factors contribute to the development of radiation dose response curves for a given animal facility with a well-defined murine model.

CONCLUSIONS

Many potential confounders that could impact the outcomes of studies to assess efficacy of a medical countermeasure for radiation-induced injuries are addressed, and recommendations are made to assist investigators in carrying out research that is robust, reproducible, and accurate.

Metabolomics in Radiation Biodosimetry: Current Approaches and Advances

Triage and medical intervention strategies for unanticipated exposure during a radiation incident benefit from the early, rapid and accurate assessment of dose level. Radiation exposure results in complex and persistent molecular and cellular responses that ultimately alter the levels of many biological markers, including the metabolomic phenotype. Metabolomics is an emerging field that promises the determination of radiation exposure by the qualitative and quantitative measurements of small molecules in a biological sample. This review highlights the current role of metabolomics in assessing radiation injury, as well as considerations for the diverse range of bioanalytical and sampling technologies that are being used to detect these changes. The authors also address the influence of the physiological status of an individual, the animal models studied, the technology and analysis employed in interrogating response to the radiation insult, and variables that factor into discovery and development of robust biomarker signatures. Furthermore, available databases for these studies have been reviewed, and existing regulatory guidance for metabolomics are discussed, with the ultimate goal of providing both context for this area of radiation research and the consideration of pathways for continued development.

Challenges and Benefits of Repurposing Products for Use during a Radiation Public Health Emergency: Lessons Learned from Biological Threats and other Disease Treatments

The risk of a radiological or nuclear public health emergency is a major growing concern of the U.S. government. To address a potential incident and ensure that the government is prepared to respond to any subsequent civilian or military casualties, the U.S. Department of Health and Human Services and the Department of Defense have been charged with the development of medical countermeasures (MCMs) to treat the acute and delayed injuries that can result from radiation exposure. Because of the limited budgets in research and development and the high costs associated with bring promising approaches from the bench through advanced product development activities, and ultimately, to regulatory approval, the U.S. government places a priority on repurposing products for which there already exists relevant safety and other important information concerning their use in humans. Generating human data can be a costly and time-consuming process; therefore, the U.S. government has interest in drugs for which such relevant information has been established (e.g., products for another indication), and in determining if they could be repurposed for use as MCMs to treat radiation injuries as well as chemical and biological insults. To explore these possibilities, the National Institute of Allergy and Infectious Diseases (NIAID) convened a workshop including U.S. government, industry and academic subject matter experts, to discuss the challenges and benefits of repurposing products for a radiation indication. Topics covered included a discussion of U.S. government efforts (e.g. funding, stockpiling and making products available for study), as well unique regulatory and other challenges faced when repurposing patent protected or generic drugs. Other discussions involved lessons learned from industry on repurposing pre-license, pipeline products within drug development portfolios. This report reviews the information presented, as well as an overview of discussions from the meeting.

Building the strategic national stockpile through the NIAID Radiation Nuclear Countermeasures Program

The possibility of a public health radiological or nuclear emergency in the United States remains a concern. Media attention focused on lost radioactive sources and international nuclear threats, as well as the potential for accidents in nuclear power facilities (e.g., Windscale, Three Mile Island, Chernobyl, and Fukushima) highlight the need to address this critical national security issue. To date, no drugs have been licensed to mitigate/treat the acute and long-term radiation injuries that would result in the event of large-scale, radiation, or nuclear public health emergency. However, recent evaluation of several candidate radiation medical countermeasures (MCMs) has provided initial proof-of-concept of efficacy. The goal of the Radiation Nuclear Countermeasures Program (RNCP) of the National Institute of Allergy and Infectious Diseases (National Institutes of Health) is to help ensure the government stockpiling of safe and efficacious MCMs to treat radiation injuries, including, but not limited to, hematopoietic, gastrointestinal, pulmonary, cutaneous, renal, cardiovascular, and central nervous systems. In addition to supporting research in these areas, the RNCP continues to fund research and development of decorporation agents targeting internal radionuclide contamination, and biodosimetry platforms (e.g., biomarkers and devices) to assess the levels of an individual's radiation exposure, capabilities that would be critical in a mass casualty scenario. New areas of research within the program include a focus on special populations, especially pediatric and geriatric civilians, as well as combination studies, in which drugs are tested within the context of expected medical care management (e.g., antibiotics and growth factors). Moving forward, challenges facing the RNCP, as well as the entire radiation research field, include further advancement and qualification of animal models, dose conversion from animal models to humans, biomarker identification, and formulation development. This paper provides a review of recent work and collaborations supported by the RNCP.

Medical countermeasures for platelet regeneration after radiation exposure. Report of a workshop and guided discussion sponsored by the National Institute of Allergy and Infectious Diseases, Bethesda, MD, March 22–23, 2010

The events of September 11, 2001 and their aftermath increased awareness of the need to develop medical countermeasures (MCMs) to treat potential health consequences of a radiation accident or deliberate attack. The medical effects of lethal exposures to ionizing radiation have been well described and affect multiple organ systems. To date, much of the research to develop treatments for mitigation of radiation-induced hematopoietic damage has focused on amelioration of radiation-induced neutropenia, which has long been considered to be the primary factor in determining survival after an unintentional radiation exposure. Consistent with historical data, recent studies have highlighted the role that radiation-induced thrombocytopenia plays in radiation mortality, yet development of MCMs to mitigate radiation damage to the megakaryocyte lineage has lagged behind anti-neutropenia approaches. To address this gap and to foster research in the area of platelet regeneration after radiation exposure, the National Institute of Allergy and Infectious Diseases (NIAID) sponsored a workshop on March 22-23, 2010 to encourage collaborations between NIAID program awardees and companies developing pro-platelet approaches. NIAID also organized an informal, open discussion between academic investigators, product development contractors, and representatives from the U.S. Food and Drug Administration (FDA) and other relevant government agencies about drug development toward FDA licensure of products for an acute radiation syndrome indication. Specific emphasis was placed on the challenges of product licensure for radiation/nuclear MCMs using current FDA regulations (21 CFR Parts 314 and 601) and on the importance of animal efficacy model development, design of pivotal protocols, and standardization of irradiation and animal supportive care.

Medical countermeasures against nuclear threats: radionuclide decorporation agents

Exposure to radionuclides disseminated by a radiological dispersion device or deposited as fallout after a nuclear power plant accident or detonation of an improvised nuclear device could result in internal contamination of a significant number of individuals. Internalized radionuclides may cause both acute and chronic radiation injury and increase an individual's risk of developing cancer. This damage and risk can be mitigated by the use of decorporation agents that reduce internal contamination. Unfortunately, most effective agents decorporate only a limited range of radionuclides, and some are formulated in ways that would make administration in mass casualty situations challenging. There is a need for new radionuclide decorporation agents, reformulations of existing agents, and/or expansion of the labeled indications for existing treatments. Researchers developing novel or improved decorporation agents should also understand the regulatory pathway for these products. This workshop, the first in nearly half a century to focus exclusively on radionuclide decorporation, brought together researchers and scientific administrators from academia, government and industry as well as senior regulatory affairs officers and U.S. Food and Drug Administration personnel. Meeting participants reviewed recent progress in the development of decorporation agents and contemplated the future of the field.

Amifostine (ETHYOL) protects rats from mucositis resulting from fractionated or hyperfractionated radiation exposure

PURPOSE

The cytoprotective drug amifostine (Ethyol) protects rats from oral mucositis resulting from a single dose of gamma-irradiation. We expanded earlier studies to determine whether multiple doses of amifostine protect against fractionated or hyperfractionated radiation and whether the active metabolite of amifostine (WR-1065) accumulates in tissues upon repeated administration.

METHODS AND MATERIALS

Rats received amifostine daily for 5 days in conjunction with a 1-week fractionated radiation schedule and were evaluated for oral mucositis. Rats also received amifostine before the am or pm exposure or b.i.d. in conjunction with hyperfractionated radiation. To determine the pharmacokinetics of WR-1065 after repeated dosing, amifostine was given 5 days a week for 1 or 3 weeks, and rat tissue and plasma were collected at intervals during and after treatment and analyzed for WR-1065.

RESULTS

Amifostine protected rats from mucositis resulting from fractionated or hyperfractionated radiation. When the number of days of amifostine administration was reduced, protection was diminished. A dose of 100 mg/kg given in the morning or 2 doses at 50 mg/kg provided the best protection against hyperfractionated radiation. WR-1065 did not accumulate in tissues or tumor upon repeated administration.

CONCLUSIONS

Amifostine prevented radiation-induced mucositis in a rat model; protection was dose and schedule dependent.

Tissue Levels of WR-1065, the Active Metabolite of Amifostine (Ethyol®), Are Equivalent following Intravenous or Subcutaneous Administration in Cynomolgus Monkeys

Amifostine (Ethyol) is a cytoprotective drug approved for the reduction of xerostomia in head and neck cancer when administered to patients receiving postoperative radiation therapy. Although amifostine is approved for intravenous infusion, the off-label subcutaneous route of administration has become more prevalent. Although human patient data indicate higher plasma bioavailability of the active metabolite (WR-1065) following intravenous compared to subcutaneous administration, there are no corresponding data showing human tissue levels of WR-1065 following either route of administration due to the difficulty in obtaining human specimens. In our study we compared plasma and tissue pharmacokinetics of WR-1065 in primates following both routes of administration. Monkeys received amifostine at a dose of 260 mg/m2 either intravenously or subcutaneously. Plasma samples were analyzed for total WR-1065 by reverse-phase high-pressure liquid chromatography (HPLC) and fluorescence detection up to 4 h after amifostine administration. Tissues were analyzed for free WR-1065 by reverse-phase HPLC and electrochemical detection 30 and 60 min after administration. Following intravenous administration, plasma WR-1065 levels peaked rapidly and showed a bi-exponential decline, while following subcutaneous administration WR-1065 levels rose slowly and declined exponentially. The relative plasma bioavailability of WR-1065 given subcutaneously was lower at 30 and 60 min. Interestingly, after 30 min, tissues showed equal or slightly greater concentrations of WR-1065 following subcutaneous administration. Levels following 60 min were comparable following both routes. The plasma bioavailability studies performed in primates confirm human plasma data. Expanding the study to evaluate primate tissue levels of WR-1065 revealed that despite lower plasma bioavailability following subcutaneous administration, tissue levels of the active metabolite were surprisingly greater than or equal to those measured in animals that received the drug intravenously. These studies strengthen the argument for subcutaneous administration of amifostine in radiation oncology.

Effects of dose and schedule on the efficacy of Ethyol: preclinical studies1 1The authors are employees and shareholders of MedImmune Inc

The chemo- and radioprotectant drug amifostine (Ethyol; MedImmune, Inc, Gaithersburg, MD) is approved for intravenous (IV) administration; however, the subcutaneous (SC) route is being explored as a practical alternative. We have previously reported equivalence between IV and SC administration using a rat model of radioprotection and active metabolite (WR-1065) tissue pharmacokinetics. To examine the more clinically relevant fractionated and hyperfractionated radiation schedules and the effects of variations in the time of amifostine administration, we expanded these studies to include radioprotection and pharmacokinetic studies of WR-1065 using multiple dosing. To measure radioprotection using a fractionated radioprotection model, rats were given amifostine over a 1-week period at various doses (25 mg/kg, 50 mg/kg, 100 mg/kg; or 162.5 mg/m(2), 325 mg/m(2), 650 mg/m(2), respectively) IV or SC daily 30 minutes before exposure to 7.5 Gy/dose. Rats were fully protected from mucositis at the highest amifostine dose, with protection diminishing as the amifostine was decreased. Equivalent protection was observed whether the drug was given IV or SC. When the number of days of amifostine administration was reduced, protection was diminished. Amifostine also protected against radiation delivered using a 1-week hyperfractionated schedule (4.5 Gy/exposure twice daily), with optimal protection occurring when the drug was administered bid 30 minutes before each exposure (50 mg/kg) or every day before the morning exposure (100 mg/kg). The need for daily dosing to achieve optimal radioprotection was consistent with the tissue pharmacokinetics of the active metabolite. We found that WR-1065 did not accumulate in tissues or in SC-implanted tumors when amifostine was administered daily for 3 weeks. In addition, tissue and tumor levels of WR-1065 declined to baseline 24 hours after each amifostine dose. In a monkey pharmacokinetic model, plasma levels of WR-1065 (characterized by a pronounced spike of WR-1065 immediately after IV administration that was absent when the drug was given SC) were similar to those of humans; however, levels of WR-1065 in the tissues were higher 30 minutes following SC administration and were equivalent 60 minutes following IV or SC administration. These results suggest that maximum tissue levels and protection occur when amifostine is given 30 to 60 minutes before radiation exposure, that treatment breaks reduce the radioprotection by amifostine, and that protection from hyperfractionated radiation is dependent on amifostine dose and schedule.

Subcutaneous administration of amifostine (ethyol) is equivalent to intravenous administration in a rat mucositis model

PURPOSE

Amifostine (Ethyol) is currently approved for intravenous (IV) administration to prevent xerostomia in patients receiving radiotherapy for head-and-neck cancer. Recently, subcutaneous (SC) administration has been explored as an alternative route. To determine whether SC administration was equivalent to IV administration, we used models to follow pharmacokinetics and oral mucosal protection in rats.

METHODS

Amifostine was administered to rats at doses of 200, 100, or 50 mg/kg (1300, 650, or 325 mg/m(2)) IV or SC at various times before radiation at 15.3 Gy (protection studies) or harvest of blood and tissues for analysis by HPLC (pharmacokinetic studies).

RESULTS

Amifostine administered IV or SC 1 h before radiation protected rats from mucositis, but the protective effect was more prolonged when amifostine was administered SC. Tissue levels of the active metabolite (WR-1065) were equivalent after SC administration. The correlation between tissue levels of WR-1065 and protection was strong, but that between blood levels of WR-1065 and protection was only weak.

CONCLUSION

These data demonstrate that, in a rat model, SC administration of amifostine was at least as effective as that by IV.

Preclinical studies on the radioprotective efficacy and pharmacokinetics of subcutaneously administered amifostine

The radioprotective effects and pharmacokinetics of subcutaneously (SC) administered amifostine have been investigated in animal studies. Studies in rats using a single dose of amifostine showed that SC administration gave protection from radiation-induced mucositis that is at least equivalent to that achieved by intravenous administration of the drug. These studies also indicate that tissue levels of the active metabolite WR-1065 correlated better with the radioprotective effects of amifostine than do plasma WR-1065 levels. Multiple-dose studies in rats show radioprotective effects equal to or greater than those obtained with intravenous dosing in the setting of fractionated irradiation. In addition, there is no evidence of drug accumulation in either normal or tumor tissue, with tumor WR-1065 levels peaking just above the limits of quantitation during treatment. Preliminary data from studies of SC amifostine in monkeys indicate a plasma pharmacokinetic profile similar to that reported earlier in humans. Tissue WR-1065 levels were higher at 30 minutes after SC dosing than they were after intravenous dosing and were comparable for the two routes at 60 minutes.

Preclinical modeling of improved amifostine (Ethyol) use in radiation therapy

Amifostine (Ethyol) has been evaluated clinically as a radioprotective agent for the prevention of xerostomia and mucositis for patients receiving radiotherapy (RT). Currently, amifostine is approved for the prevention of xerostomia in head and neck cancer patients receiving RT when administered intravenously (IV) before RT. For the clinician, there would be several advantages to administering the drug subcutaneously and to being able to show its protective effects on mucositis. The authors have developed a rat RT model to examine the protective effects of amifostine after IV and subcutaneous (SC) administration in a mucositis model. Rats (5 per group) were given 200 mg/kg (human dose equivalent of approximately 1,300 mg/m(2)) of amifostine either IV or SC, and their head and neck regions were exposed to 15.3 Gy of gamma radiation 0.5, 2, 4, and 8 hours after amifostine administration. For 10 days after treatment, the oral cavities of the rats were examined for signs of mucositis. Mucosal erythema and mucosal edema were scored according to 0 through 5 and 0 through 2 scales, respectively, with the scores added to indicate overall mucositis. The average mucositis score for the untreated animals was 3.5. Rats were protected from mucositis up to 4 hours when given amifostine either IV or SC. Rats that received amifostine SC, but not IV, were protected from mucositis 8 hours after administration. Preliminary pharmacokinetic data have revealed slightly higher active metabolite (WR-1065) levels in the parotid gland and small intestine in the rats given amifostine SC compared with IV and equivalent levels in the plasma and kidney. The data showed that SC administration of amifostine gave radioprotection comparable to IV administration up to 4 hours before RT and may be more effective than IV administration at longer pretreatment intervals.

Conformational nature of the Borrelia burgdorferi decorin binding protein A epitopes that elicit protective antibodies

Decorin binding protein A (DbpA) has been shown by several laboratories to be a protective antigen for the prevention of experimental Borrelia burgdorferi infection in the mouse model of Lyme borreliosis. However, different recombinant forms of the antigen having either lipidated amino termini, approximating the natural secretion and posttranslational processing, or nonprocessed cytosolic forms have elicited disparate levels of protection in the mouse model. We have now used the unique functional properties of this molecule to investigate the structural requirements needed to elicit a protective immune response. Genetic and physicochemical alterations to DbpA showed that the ability to bind to the ligand decorin is indicative of a potent immunogen but is not conclusive. By mutating the two carboxy-terminal nonconserved cysteines of DbpA from B. burgdorferi strain N40, we have determined that the stability afforded by the putative disulfide bond is essential for the generation of protective antibodies. This mutated protein was more sensitive to thermal denaturation and proteolysis, suggesting that it is in a less ordered state. Immunization with DbpA that was thermally denatured and functionally inactivated stimulated an immune response that was not protective and lacked bactericidal antibodies. Antibodies against conformationally altered forms of DbpA also failed to kill heterologous B. garinii and B. afzelii strains. Additionally, nonsecreted recombinant forms of DbpA(N40) were found to be inferior to secreted lipoprotein DbpA(N40) in terms of functional activity and antigenic potency. These data suggest that elicitation of a bactericidal and protective immune response to DbpA requires a properly folded conformation for the production of functional antibodies.

Evidence for vaccine synergy between Borrelia burgdorferi decorin binding protein A and outer surface protein A in the mouse model of lyme borreliosis

Mice immunized with either the predominantly vector-stage lipoprotein outer surface protein A (OspA) or the in vivo-expressed lipoprotein decorin binding protein A (DbpA) are protected against Borrelia burgdorferi challenge. DbpA-OspA combinations protected against 100-fold-higher challenge doses than did either single-antigen vaccine and conferred significant protection against heterologous B. burgdorferi, B. garinii, and B. afzelii isolates, suggesting that there is synergy between these two immunogens.

DbpA, but not OspA, is expressed by Borrelia burgdorferi during spirochetemia and is a target for protective antibodies

DbpA is a target for antibodies that protect mice against infection by cultured Borrelia burgdorferi. Infected mice exhibit early and sustained humoral responses to DbpA and DbpB, suggesting that these proteins are expressed in vivo. Many antigens expressed in mammals by B. burgdorferi are repressed in vitro at lower growth temperatures, and we have now extended these observations to include DbpA and DbpB. To confirm that the protective antigen DbpA is expressed in vivo and to address the question of its accessibility to antibodies during infection, we examined B. burgdorferi in blood samples from mice following cutaneous inoculation. B. burgdorferi was visualized by dark-field microscopy in plasma samples from spirochetemic mice, and an indirect immunofluorescence assay showed that these spirochetes were DbpA positive and OspA negative. We developed an ex vivo borreliacidal assay to show that hyperimmune antiserum against DbpA, but not OspA, killed these plasma-derived spirochetes, demonstrating that DbpA is accessible to antibodies during this phase of infection. Blood transferred from spirochetemic donor mice readily established B. burgdorferi infection in naive recipient mice or mice hyperimmunized with OspA, while mice hyperimmunized with DbpA showed significant protection against challenge with host-adapted spirochetes. Antiserum from persistently infected mice had borreliacidal activity against both cultured and plasma-derived spirochetes, and adsorption of this serum with DbpA substantially depleted this killing activity. Our observations show that immunization with DbpA blocks B. burgdorferi dissemination from the site of cutaneous inoculation and suggest that DbpA antibodies may contribute to control of persistent infection.

Active and passive immunity against Borrelia burgdorferi decorin binding protein A (DbpA) protects against infection

Borrelia burgdorferi, the spirochete that causes Lyme disease, binds decorin, a collagen-associated extracellular matrix proteoglycan found in the skin (the site of entry for the spirochete) and in many other tissues. Two borrelial adhesins that recognize this proteoglycan, decorin binding proteins A and B (DbpA and DbpB, respectively), have recently been identified. Infection of mice by low-dose B. burgdorferi challenge elicited antibodies against DbpA and DbpB that were sustained at high levels, suggesting that these antigens are expressed in vivo. Scanning immunoelectron microscopy showed that DbpA was surface accessible on intact borreliae. Passive administration of DbpA antiserum protected mice from infection following challenge with heterologous B. burgdorferi sensu stricto isolates, even when serum administration was delayed for up to 4 days after challenge. DbpA is the first antigen target identified that is capable of mediating immune resolution of early, localized B. burgdorferi infections. DbpA immunization also protected mice from B. burgdorferi challenge; DbpB immunization was much less effective. DbpA antiserum inhibited in vitro growth of many B. burgdorferi sensu lato isolates of diverse geographic, phylogenetic, and clinical origins. In combination, these findings support a role for DbpA in the immunoprophylaxis of Lyme disease and suggest that DbpA vaccines have the potential to eliminate early-stage B. burgdorferi infections.

Antibody and cytotoxic T-lymphocyte responses to a single liposome-associated peptide antigen

The development of peptide-based vaccines that elicit antibody (Ab) and cellular immune responses has been hampered by the lack of highly immunogenic formulations. In this study, we compared the induction of Ab and cytotoxic T-lymphocyte (CTL) responses to a peptide derived from the V3 loop of HIV-1 gp120 (P18 and its cysteine-glycine derivative (CG-P18)) when incorporated into liposomes with lipid A (LA) or mixed with aluminum hydroxide. P18-specific CTL were only observed with liposomes with LA. P18-specific Ab responses were found with liposomes containing CG-P18 but not P18. Increased surface expression of the former, resulted in enhancement of the Ab response without loss of CTL induction. Thus, the manner in which a peptide is localized can influence the outcome of the response induced by highly immunogenic liposome formulations.

Immunization with soluble murine CD4 induces an anti-self antibody response without causing impairment of immune function

The T cell surface molecule CD4 (L3T4 in mouse) is important in the T lymphocyte response to Ag presented in association with MHC class II molecules. To examine the role of CD4 in immune function, we expressed a soluble form of murine CD4 by deleting the transmembrane and cytoplasmic regions of the L3T4 gene and transfecting the altered gene into Chinese hamster ovary cells. The recombinant soluble mouse CD4 (smCD4) retained the native conformation of the external portion, as indicated by the binding of L3T4 mAb. In vitro, smCD4 did not inhibit class II-dependent, Ag-specific, T cell proliferation or MLR, even at concentrations 300-fold greater, on a molar basis, than that of anti-CD4 mAb. Immunization of mice with smCD4 induced a strong anti-CD4 response. These antibodies showed some binding to native cell surface CD4, indicating that immunization with smCD4 generated an anti-self response. Analysis of lymphoid cells from spleen, lymph node, and thymus of smCD4-treated mice revealed no alteration in subset phenotypes. Also, Th cell function, as measured by response to soluble Ag, was not compromised. Thus, smCD4 did not inhibit T cell activity in vitro, and the autoimmune response arising from immunization with smCD4 had no apparent consequences for normal immune function.

Immunization with soluble murine CD4 induces an anti-self antibody response without causing impairment of immune function

The T cell surface molecule CD4 (L3T4 in mouse) is important in the T lymphocyte response to Ag presented in association with MHC class II molecules. To examine the role of CD4 in immune function, we expressed a soluble form of murine CD4 by deleting the transmembrane and cytoplasmic regions of the L3T4 gene and transfecting the altered gene into Chinese hamster ovary cells. The recombinant soluble mouse CD4 (smCD4) retained the native conformation of the external portion, as indicated by the binding of L3T4 mAb. In vitro, smCD4 did not inhibit class II-dependent, Ag-specific, T cell proliferation or MLR, even at concentrations 300-fold greater, on a molar basis, than that of anti-CD4 mAb. Immunization of mice with smCD4 induced a strong anti-CD4 response. These antibodies showed some binding to native cell surface CD4, indicating that immunization with smCD4 generated an anti-self response. Analysis of lymphoid cells from spleen, lymph node, and thymus of smCD4-treated mice revealed no alteration in subset phenotypes. Also, Th cell function, as measured by response to soluble Ag, was not compromised. Thus, smCD4 did not inhibit T cell activity in vitro, and the autoimmune response arising from immunization with smCD4 had no apparent consequences for normal immune function.

A region in domain 1 of CD4 distinct from the primary gp120 binding site is involved in HIV infection and virus-mediated fusion

The high affinity binding site for human immunodeficiency virus (HIV) envelope glycoprotein gp120 resides within the amino-terminal domain (D1) of CD4. Mutational and antibody epitope analyses have implicated the region encompassing residues 40-60 in D1 as the primary binding site for gp120. Outside of this region, a single residue substitution at position 87 abrogates syncytium formation without affecting gp120 binding. We describe two groups of CD4 monoclonal antibodies (mAbs) which recognize distinct epitopes associated with these regions in D1. These mAbs distinguish between the gp120 binding event and virus infection and virus-induced cell fusion. One cluster of mAbs, which bind at or near the high affinity gp120 binding site, blocked gp120 binding to CD4 and, as expected, also blocked HIV infection of CD4+ cells and virus-induced syncytium formation. A second cluster of mAbs, which recognize the CDR-3 like loop, did not block gp120 binding as demonstrated by their ability to form ternary complexes with CD4 and gp120. Yet, these mAbs strongly inhibited HIV infection of CD4+ cells and HIV-envelope/CD4-mediated syncytium formation. The structure of D1 has recently been solved at atomic resolution and in its general features resembles IgVk regions as predicted from sequence homology and mAb epitopes. In the D1 structure, the regions recognized by these two groups of antibodies correspond to the C'C" (Ig CDR2) and FG (Ig CDR3) hairpin loops, respectively, which are solvent-exposed beta turns protruding in two different directions on a face of D1 distal to the D2 domain. This face is straddled by the longer BC (Ig CDR1) loop which bisects the plain formed by C'C'' and FG. This structure is consistent with C'C'' and FG forming two distinct epitope clusters within D1. We conclude that the initial interaction between gp120 and CD4 is not sufficient for HIV infection and syncytium formation and that CD4 plays a critical role in the subsequent virus-cell and cell-cell membrane fusion events. We propose that the initial binding of CD4 to gp120 induces conformational changes in gp120 leading to subsequent interactions of the FG loop with other regions in gp120 or with the fusogenic gp41 potion of the envelope gp160 glycoprotein.

Artificial T cell:B cell conjugation. A unique approach to analyze weak cell-cell interactions

An antibody response against a thymic-dependent Ag requires cognate recognition of the Ag by B and T cells. Functional T-B cell (T-B) interaction involves binding of Ag by B cell surface Ig, internalization and processing of Ag, expression of an Ag fragment in the context of Ia, binding of Ag/Ia by the TCR and binding of T cell-derived lymphokines by B cell lymphokine receptors. It is becoming increasingly evident that B and T cell accessory molecules also are involved in T-B interactions. To determine the role of accessory molecules in T-B collaboration, we have designed a system in which T-B interaction was artificially induced in the absence of carrier protein. TNP-modified, turkey gamma-globulin-specific, Th cells were allowed to form conjugates with TNP-specific B cells in the absence of hapten-carrier complex. Both B and T cells were induced to proliferate and B cells partially differentiated into antibody-secreting cells when B cells were cultured with TNP-modified but not unmodified T cells. The activation of B cells by TNP-modified T cells was not MHC restricted but was blocked by anti-Ia antibodies, suggesting a role for Ia distinct from Ag presentation. Furthermore, B cell proliferation was also inhibited by antibodies to L3T4 and LFA-1, suggesting a functional accessory role for these molecules in induction of B cell proliferation/differentiation.

Artificial T cell:B cell conjugation. A unique approach to analyze weak cell-cell interactions

An antibody response against a thymic-dependent Ag requires cognate recognition of the Ag by B and T cells. Functional T-B cell (T-B) interaction involves binding of Ag by B cell surface Ig, internalization and processing of Ag, expression of an Ag fragment in the context of Ia, binding of Ag/Ia by the TCR and binding of T cell-derived lymphokines by B cell lymphokine receptors. It is becoming increasingly evident that B and T cell accessory molecules also are involved in T-B interactions. To determine the role of accessory molecules in T-B collaboration, we have designed a system in which T-B interaction was artificially induced in the absence of carrier protein. TNP-modified, turkey gamma-globulin-specific, Th cells were allowed to form conjugates with TNP-specific B cells in the absence of hapten-carrier complex. Both B and T cells were induced to proliferate and B cells partially differentiated into antibody-secreting cells when B cells were cultured with TNP-modified but not unmodified T cells. The activation of B cells by TNP-modified T cells was not MHC restricted but was blocked by anti-Ia antibodies, suggesting a role for Ia distinct from Ag presentation. Furthermore, B cell proliferation was also inhibited by antibodies to L3T4 and LFA-1, suggesting a functional accessory role for these molecules in induction of B cell proliferation/differentiation.

The requirement for surface Ig signaling as a prerequisite for T cell:B cell interactions. A possible role for desialylation

Models for T cell:B cell collaboration suggest that activated B cells process and present Ag to Th cells which subsequently induce B cell proliferation and differentiation. In contrast to activated B cells, resting B cells have generally been shown to be less efficient APC. If this model of T:B collaboration is physiologically correct, then resting B cells must undergo a phenotypic change that permits effective interaction with T cells. In this report, the requirement for rapid signaling through surface Ig on resting B cells for the induction of T:B interaction was investigated with an in vitro clustering assay. Resting splenic B cells were unable to form specific conjugates with T cell clones, unless the B cells were first treated with neuraminidase to remove sialic acid. In contrast, LPS-activated B cells were able to form conjugates without prior treatment. The ability of antibody against LFA-1 or L3T4 to inhibit cluster formation depended on the state of B cell activation in that anti-LFA-1 and anti-L3T4 mAb inhibited cluster formation by neuraminidase-treated resting B cells, but not by LPS-activated B cells. In addition, Ag-specific B cells which were isolated by their capacity to bind specific Ag were able to form clusters without any additional treatment. Moreover, treatment of resting splenic B cells with anti-mu-antibody induced clustering potential in B cells in as little as 10 min, suggesting that signaling through surface Ig was sufficient to induce this phenotypic change in B cells. Furthermore, activation of protein kinase C and Ca2+ mobilization were shown to be involved in that PMA and ionomycin treatment were also able to induce clustering potential in resting B cells. The rapid induction of clustering potential in resting B cells after signaling through surface Ig may represent a fundamental change in B cell physiology which occurs after recognition of specific Ag and may be required for effective cognate recognition between resting hapten-specific B cells and carrier-specific T cells. The potential role of desialylation for the induction of T:B interaction is discussed.

The requirement for surface Ig signaling as a prerequisite for T cell:B cell interactions. A possible role for desialylation

Models for T cell:B cell collaboration suggest that activated B cells process and present Ag to Th cells which subsequently induce B cell proliferation and differentiation. In contrast to activated B cells, resting B cells have generally been shown to be less efficient APC. If this model of T:B collaboration is physiologically correct, then resting B cells must undergo a phenotypic change that permits effective interaction with T cells. In this report, the requirement for rapid signaling through surface Ig on resting B cells for the induction of T:B interaction was investigated with an in vitro clustering assay. Resting splenic B cells were unable to form specific conjugates with T cell clones, unless the B cells were first treated with neuraminidase to remove sialic acid. In contrast, LPS-activated B cells were able to form conjugates without prior treatment. The ability of antibody against LFA-1 or L3T4 to inhibit cluster formation depended on the state of B cell activation in that anti-LFA-1 and anti-L3T4 mAb inhibited cluster formation by neuraminidase-treated resting B cells, but not by LPS-activated B cells. In addition, Ag-specific B cells which were isolated by their capacity to bind specific Ag were able to form clusters without any additional treatment. Moreover, treatment of resting splenic B cells with anti-mu-antibody induced clustering potential in B cells in as little as 10 min, suggesting that signaling through surface Ig was sufficient to induce this phenotypic change in B cells. Furthermore, activation of protein kinase C and Ca2+ mobilization were shown to be involved in that PMA and ionomycin treatment were also able to induce clustering potential in resting B cells. The rapid induction of clustering potential in resting B cells after signaling through surface Ig may represent a fundamental change in B cell physiology which occurs after recognition of specific Ag and may be required for effective cognate recognition between resting hapten-specific B cells and carrier-specific T cells. The potential role of desialylation for the induction of T:B interaction is discussed.