Species comparison of anatomical and functional immune system development

Lead and immune function

The heavy metal lead is a widely deposited environmental toxicant known to impact numerous physiological systems, including the reproductive, neurological, hepatic, renal, and immune systems. Studies illustrating the capacity of lead to impair immune function and/or host resistance to disease date back to at least the 1960s. However, it has only been in recent years that lead has been recognized among a new category of immunotoxicants-those that dramatically shift immune functional capacity while producing only modest changes to immune cell populations and lymphoid organs. These relatively noncytotoxic immunomodulating chemicals and drugs represent the immunotoxic hazards most difficult to identify and problematic for risk assessment using historic approaches. As a result, such environmental factors are also among the most likely to contribute to chronic immune-related disease at relevant exposure levels. This review considers the animal and human evidence that lead exposure can produce a stark shift in immune functional capacity with a skewing predicted to elevate the risk of atopic and certain autoimmune diseases. At the same time, host defenses against infectious agents and cancer may be reduced. Age-based exposure studies also suggest that levels of blood lead previously thought to be safe, that is, below 10 microg/dl, may be associated with later life immune alterations.

Perinatal immunotoxicity: why adult exposure assessment fails to predict risk

Recent research has pointed to the developing immune system as a remarkably sensitive toxicologic target for environmental chemicals and drugs. In fact, the perinatal period before and just after birth is replete with dynamic immune changes, many of which do not occur in adults. These include not only the basic maturation and distribution of immune cell types and selection against autoreactive lymphocytes but also changes designed specifically to protect the pregnancy against immune-mediated miscarriage. The newborn is then faced with critical immune maturational adjustments to achieve an immune balance necessary to combat myriad childhood and later-life diseases. All these processes set the fetus and neonate completely apart from the adult regarding immunotoxicologic risk. Yet for decades, safety evaluation has relied almost exclusively upon exposure of the adult immune system to predict perinatal immune risk. Recent workshops and forums have suggested a benefit in employing alternative exposures that include exposure throughout early life stages. However, issues remain concerning when and where such applications might be required. In this review we discuss the reasons why immunotoxic assessment is important for current childhood diseases and why adult exposure assessment cannot predict the effect of xenobiotics on the developing immune system. It also provides examples of developmental immunotoxicants where age-based risk appears to differ. Finally, it stresses the need to replace adult exposure assessment for immune evaluation with protocols that can protect the developing immune system.

B-cell Tolerance following ABO-Incompatible Infant Heart Transplantation

Infant heart transplantation is highly successful therapy for severe congenital heart malformations and cardiomyopathies. However the donor organ shortage is particularly crucial for this age group. We have demonstrated that some aspects of human immaturity can be exploited for infant organ transplantation, allowing safe transplantation of organs that would be "incompatible" in mature individuals, and as a window of continued susceptibility to induction of immunologic tolerance.

Developmental immunotoxicity investigations in the SD rat following pre- and post-natal exposure to cyclosporin

BACKGROUND

The development and function of the immune system was assessed in juvenile SD rats following pre- or post-natal exposure to cyclosporin. The main objective was to assess the feasibility of the methods available for the detection of adverse effects on the development of the immune system for use in the safety assessment of medicines.

METHODS

In a pre-natal experiment, 15 pregnant rats were given 10 mg/kg/day of cyclosporin by gavage from day 6 of gestation until 4 days after parturition. A control group received olive oil. In a post-natal experiment, the pups from 35 litters were given 10 mg/kg/day of cyclosporin by gavage from 4 to 28 days of age. Half of the pups in each litter were given water and acted as controls. Immune endpoints were determined in the pups in both experiments from two to 10 weeks of age, including: lymphocyte subsets, serum immunoglobulin titres, serum autoantibodies, primary antibody response to sheep red blood cells (SRBC), delayed-type hypersensitivity response, humoral response to keyhole limpet haemocyanin, spleen cellularity, immune organ weights, and histopathology.

RESULTS

Pre-natal exposure caused no effects on immune function. Post-natal exposure caused immune depression during the treatment period and a persistent impairment of the immune system characterised by lymphoid hyperplasia in the spleen and a reduced primary antibody response to SRBC at 10 weeks of age.

CONCLUSIONS

These results demonstrate the importance of a post-treatment follow-up period in developmental immunotoxicity studies, in order to distinguish between the transient effects of immune modulation and the persistent consequences of developmental toxicity.

Immune assessments in developmental and juvenile toxicology: Practical considerations for the regulatory safety testing of pharmaceuticals

The developing organism is considered to be more sensitive than the adult to immunotoxic agents. There is every reason, therefore, to include immune assessments in the regulatory testing for developmental toxicity of drugs that are intended to be used in young patients or pregnant woman. An effective strategy would be to incorporate immune assessments in the existing recommendations on pre- and post-natal toxicity study in the rat from the International Conference on Harmonisation. Immune assessments could also be included in juvenile toxicity studies to screen for effects resulting from post-natal exposure to the drug. Adequate testing methods are available to screen for developmental effects that result in immune depression. Routine immune assessments may comprise histopathological examination of the lymphoid organs/tissues and immunophenotyping of lymphocyte subsets in the blood, spleen, or thymus. These tests should be performed in rodents at various ages and at various stages of pre- and post-weaning development. Immunoglobulin and cytokine measurements, assessment of the T-cell dependent antigen response to sheep red blood cells or keyhole limpet haemocyanin antigens, and host resistance studies may be performed as apical tests at maturity. More research is required to develop methods for the detection of drugs that may render the developing organism more susceptible to hypersensitivity or autoimmunity.

Assessing susceptibility from early-life exposure to carcinogens

Cancer risk assessment methods currently assume that children and adults are equally susceptible to exposure to chemicals. We reviewed available scientific literature to determine whether this was scientifically supported. We identified more than 50 chemicals causing cancer after perinatal exposure. Human data are extremely limited, with radiation exposures showing increased early susceptibility at some tumor sites. Twenty-seven rodent studies for 18 chemicals had sufficient data after postnatal and adult exposures to quantitatively estimate potential increased susceptibility from early-life exposure, calculated as the ratio of juvenile to adult cancer potencies for three study types: acute dosing, repeated dosing, and lifetime dosing. Twelve of the chemicals act through a mutagenic mode of action. For these, the geometric mean ratio was 11 for lifetime exposures and 8.7 for repeat exposures, with a ratio of 10 for these studies combined. The geometric mean ratio for acute studies is 1.5, which was influenced by tissue-specific results [geometric mean ratios for kidney, leukemia, liver, lymph, mammary, nerve, reticular tissue, thymic lymphoma, and uterus/vagina > 1 (range, 1.6-8.1); forestomach, harderian gland, ovaries, and thyroid < 1 (range, 0.033-0.45)]. Chemicals causing cancer through other modes of action indicate some increased susceptibility from postnatal exposure (geometric mean ratio is 3.4 for lifetime exposure, 2.2 for repeat exposure). Early exposures to compounds with endocrine activity sometimes produce different tumors after exposures at different ages. These analyses suggest increased susceptibility to cancer from early-life exposure, particularly for chemicals acting through a mutagenic mode of action.

In utero gene therapy: current challenges and perspectives

Over the past few years, considerable progress in prenatal diagnosis and surgery combined with improvements in vector design vindicate a reappraisal of the feasibility of in utero gene therapy for serious monogenetic diseases. As adult gene therapy gathers pace, several apparent obstacles to its application as a treatment may be overcome by pre- or early postnatal treatment. This review will examine the concepts and practice of prenatal vector administration. We aim to highlight the advantages of early therapeutic intervention focusing on diseases that could benefit greatly from a prenatal gene therapy approach. We will pay special attention to the strategies and vectors that are most likely to be used for this application and will speculate on their expected developments for the near future.

Computational pharmacokinetics during developmental windows of susceptibility

Computational modeling has an increasing role in analyses of biological effects, including how the body handles chemicals (i.e., pharmacokinetics or toxicokinetics) and how the body responds to chemicals (i.e., pharmacodynamics or toxicodynamics). Pharmacokinetic models increasingly describe not just adult humans and animals, but also changes with age and life stage (e.g., pregnancy and fetal exposures, lactational exposures, and childhood growth). Physiologically based pharmacokinetic models provide an important route to estimate the potential changes in internal dose that may occur throughout the life cycle. These models require inputs describing changes in physiology, metabolism, and exposure with age and life stage. A particular challenge exists when the "equivalent" developmental period in the rodents and humans differs (e.g., early postnatal in rats and in utero in humans) such that the "equivalent" window of susceptibility to toxic effects of the chemical may involve substantially different exposures (e.g., lactational versus placental transfer). Pharmacodynamic modeling could similarly address changes with age, but few such models currently exist. The growth of systems biology is anticipated to change this over the coming decade.

Immune function testing of human pharmaceuticals: regulatory overshoot?

Suppression of the activity of the immune system can result in decreased resistance to (opportunistic) infection and cancer. Experimental animal studies have identified many immunotoxic agents, but data on unintended immuno-toxicity of human pharmaceuticals are limited. To enhance the sensitivity of the non-clinical screening of human pharmaceuticals, immunotoxicity endpoints in standard toxicity studies, as well as an immune function study, are requested by the EU, incorporating a T-cell-dependent antigen response in the first screening phase. Authorities from other areas in the world follow similar lines for immunotoxicity testing, but differ in whether or not the functional assay is seen as a primary requisite or can be secondary to other factors or observations. Based on a limited survey in the framework of the International Conference on Harmonization, the European position has been reconsidered to negotiate a broad cause-for-concern approach. The relevance of functional testing of the immune system to show effects of human pharmaceuticals is shown in this review. There is certainly a need for more in-depth testing of the integrity of the immune system and a cause-for-concern approach is, therefore, needed. A broad category is the number of compounds that might interact via binding to receptor sites at the broadly taken population of leukocytes (e.g., lymphocytes, macrophages). Criteria to differentiate among compounds in this category have yet to be ascertained.

Donor-specific B-cell tolerance after ABO-incompatible infant heart transplantation

Although over 50 years have passed since its first laboratory description, intentional induction of immune tolerance to foreign antigens has remained an elusive clinical goal. We previously reported that the requirement for ABO compatibility in heart transplantation is not applicable to infants. Here, we show that ABO-incompatible heart transplantation during infancy results in development of B-cell tolerance to donor blood group A and B antigens. This mimics animal models of neonatal tolerance and indicates that the human infant is susceptible to intentional tolerance induction. Tolerance in this setting occurs by elimination of donor-reactive B lymphocytes and may be dependent upon persistence of some degree of antigen expression. These findings suggest that intentional exposure to nonself A and B antigens may prolong the window of opportunity for ABO-incompatible transplantation, and have profound implications for clinical research on tolerance induction to T-independent antigens relevant to xenotransplantation.

Developing drugs for pediatric use: a role for juvenile animal studies?

The increased interest and/or need to perform pediatric clinical trials to allow the marketing and safe use of a wider range of medicines in children has raised the profile of the need to conduct juvenile animal studies. It is argued that such studies may identify "unique" toxicities not seen from available adult animal and clinical data. This paper will review the current situation from an industrial, regulatory, and scientific perspective. Areas of important consideration include functional (dynamic) and kinetic differences between children and adults and specific considerations associated with testing in young animals. This paper will also review what are we currently doing? Whether we really need these studies? and What challenges lie in the future?

Symposium summary: children's health risk--what's so special about the developing immune system?

In recent years, there has been increasing regulatory pressure to protect the health of children, with the basic tenet being that children differ significantly from adults in their biological or physiological responses to chemical exposures. In a regulatory context, this has been translated to mean a requirement for an additional 10-fold safety factor for environmental contaminants, specialized tests, or both. Much of the initial focus has been on the developing endocrine and nervous systems; but increasingly, the developing immune system has been identified as a potential target organ for chemically mediated toxicity. More recently, the question has been raised regarding whether the current state of science supports the creation of developmental immunotoxicology (DIT) test guidelines. What is needed is a risk-based evaluation of the biology associated with the proposed differential sensitivity between children and adults and the impact of that assessment on additional regulatory measures to protect children in risk assessment analyses. Additionally, an understanding of whether the developing immune system shows greater susceptibility, either qualitatively or quantitatively, to chemical perturbation is critical. To address the question "What's so special about the developing immune system?" a symposium was organized for the 2003 Society of Toxicology annual meeting that brought together risk assessors, clinicians, immunologists, and toxicologists.