Immunosuppression testing

Many attempts are underway to devise in vitro assays for immunotoxicity testing. The first results will likely comprise assays with limited applicability that may still be very usefijl in the context of prescreening (e.g., for pharmaceutical companies) and in situations where there is limited information. For immunosuppression testing prevalidation studies are at the brink of being started. For sensitization testing several routes (biochemical, cell culture, in silico) are being pursued. For testing other types of immunotoxicity, in vitro alternatives are not foreseen within the next decade. 

Corticotropin is used for diagnostic testing of adrenocortical function. This drug may also be used for the management of acute exacerbations of multiple sclerosis, nonsuppurative thyroiditis, and hypercalcemia associated with cancer. It is also used as an anti-inflammatory and immunosuppressant drug when conventional glucocorticoid therapy lias not been effective (see Display 50-1). 

Since the limited information available on the effects of dermally administered endosulfan suggests that this chemical behaves similarly across both routes of exposure and that adverse effects on immune function end points have also been observed in vitro, there is no reason to suspect that the immunotoxic effects observed following oral exposure are route-specific. Tests of immunologic function in exposed human populations would provide information as to whether immunosuppression also occurs in humans... 

Cyclosporine is a cyclic polypeptide immunosuppressant typically used to prevent organ rejection in transplant patients. Its use is restricted to patients with fulminant or refractory symptoms in patients with active IBD. Significant toxicides associated with cyclosporine are nephrotoxicity, risk of infection, seizures, hypertension, and liver function test abnormalities.1,13,14... 

Nevertheless, there are reports on enhancement of ocular drug absorption by bile salts [33], surfactants [200], and chelators [149], Newton et al. [35] demonstrated that Azone, an enhancer widely tested in transdermal drug delivery [201], increased the ocular absorption of cyclosporine, an immunosuppressant, by a factor of 3, thereby prolonging the survival of a corneal allograft. In 1986, Lee et al. [34] reported that 10 pg/mL cytochalasin B, an agent capable of condensing the actin microfilaments, increased the aqueous humor and iris-ciliary body concentrations of topically applied inulin (5 kDa) by about 70% and 700%, respectively, in the albino rabbit. 

Selective immunosuppression in individuals suffering from the above conditions is likely best achieved by preventing the synthesis or functioning of IL-2. Cyclosporin A, one of the foremost immunosuppressive agents currently in use, functions by preventing IL-2 synthesis. A number of alternative approaches are now being considered or tested directly in clinical trials. These include ... 

Cyclosporine demonstrates immunosuppressive activity by inhibiting the first phase of T-cell activation. It also inhibits release of inflammatory mediators from mast cells, basophils, and polymorphonuclear cells. It is used in the treatment of both cutaneous and arthritis manifestations of severe psoriasis. The usual dose is between 2.5 and 5 mg/kg/day given in two divided doses. Adverse effects include nephrotoxicity, hypertension, hypomagnesemia, hyperkalemia, alterations in liver function tests, elevations of serum lipids, GI intolerance, paresthesias, hypertrichosis, and gingival hyperplasia. Cumulative treatment for more than 2 years may increase the risk of malignancy, including skin cancers and lymphoproliferative disorders. 

The majority of early publications that can be reasonably identified as comprising immunotoxicology reported altered resistance to infection in animals exposed to various environmental or industrial chemicals. Authors logically concluded that xenobiotic exposure suppressed immune function since the immune system is ultimately responsible for this resistance to infection. Subsequent studies demonstrated that suppression of various cellular and functional endpoints accompanied or preceded increased sensitivity to infection, and that administration of known immunosuppressants likewise decreased host resistance. The human health implications of these studies, that chemical exposure reduced resistance to infection, drove the initial focus of many immunotoxicologists on functional suppression, and provided the theoretical and practical underpinnings of immunotoxicity testing. 

While changes in cell phenotypes have proved useful in some settings to characterize the immunotoxicity of xenobiotics,1 phenotypic analysis alone is often not a sensitive indicator of low dose immunotoxicity for many agents that alter immune function. Xenobiotics that exert selective toxicity on lymphoid and myeloid cells may be discovered through immunophenotypic analysis. However, most agents produce immunotoxicity at doses much lower than those required to produce cytotoxicity or interfere with primary lymphoid organ differentiation. Some of the most potent immunosuppressive chemicals that have been tested, such as cyclosporine A, do not alter immunophenotype at doses that are immunosuppressive. On the other hand, when phenotyping is linked to assessment of functional parameters of the cells, immunotoxic effects are more likely to be identified. 

Immunotoxicity testing in rodents exposed to industrial and/or environmental chemicals, has been recognized as an important toxicological concern for over 25 years. Early immunotoxicity testing relied primarily on the mouse, due to the plethora of immune structure and function research performed by immunologists to better understand the human immune system. As such, the mouse has been the most employed rodent for immunotoxicity testing. Immune system function assays employed in screening for immunotoxicity were developed in adult mice. These same immune function assays have served to help identify toxicant induced immunosuppression in the rat. 

A review of the literature on chemical-induced immunosuppression in rats and mice, exposed during the pre- and/or postnatal period, was compared to exposure of adults. Five known immunosuppressants (i.e., TCDD, TBTO, DES, Pb, and diazepam) were reviewed. The data revealed that the developing immune system was more sensitive to chemical exposure than the mature immune system. Based on these evaluations, the authors concluded that it was reasonable to assume that testing only in adults would not provide a sufficient level of sensitivity to define immunotoxicity in the neonate 132. In summary, this chapter provides compelling evidence that the developing, compared to the mature, immune system is more vulnerable to perturbation. 

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