Diphenylmethane diisocyanate (MDI)-induced asthma: evaluation of the immunologic responses and application of an animal model of isocyanate sensitivity

Molecular determinants of humoral immune specificity for the occupational allergen, methylene diphenyl diisocyanate

Methylene diphenyl diisocyanate (MDI), a low molecular weight chemical important for producing polyurethane foam, coatings, and elastomers is a major cause of occupational asthma, however, mechanisms of disease pathogenesis remain poorly understood. This study characterizes the rearranged germline and hypervariable region cDNA of new anti-MDI secreting hybridomas derived from mice immunized with MDI-conjugated to autologous serum proteins. Six IgG1 secreting clones were identified in initial screening ELISAs, based on differential binding to MDI conjugated human albumin vs. mock exposed albumin. The mAbs secreted by the hybridomas also recognized MDI conjugated to other model proteins (e.g. ovalbumin, transferrin), but did not bind unconjugated proteins, or protein conjugates prepared with other isocyanates (e.g. TDI, HDI). The mAbs displayed MDI-dose dependent binding in ELISA and Western blot, and exhibited varying degrees of cross-competition, suggesting differences in epitope specificity. The cDNA encoding the monoclonal antibodies reveal clonal differences in the CDR3 regions, germline gene usage, and patterns of somatic hypermutation related to epitope specificity. Together, the data provide new insight into the molecular determinants of humoral MDI specificity, and characterize anti-MDI IgG1 mAbs that may be developed into useful diagnostic reagents.

Cytokine mRNA profiles for isocyanates with known and unknown potential to induce respiratory sensitization

Isocyanates are low-molecular-weight chemicals implicated in allergic asthmatic-type reactions. Identification of chemicals likely to cause asthma is difficult due to the lack of a validated test method. One hypothesis is that differential cytokine induction (Th1 versus Th2 profiles) in the draining lymph node following dermal application can be used to identify asthmagens and distinguish them from contact allergens. In this study, we compared the cytokine mRNA profiles of six chemicals: toluene diisocyanate (TDI), diphenylmethane-4,4'-diisocyanate (MDI), dicyclohexylmethane-4,4'-diisocyanate (HMDI), isophorone diisocyanate (IPDI), p-tolyl(mono)isocyanate (TMI), and meta-tetramethylene xylene diisocyanate (TMXDI). Whereas TDI and MDI are well-known respiratory sensitizers, documentation for HMDI, IPDI, TMI, and TMXDI is limited, but suggests that HMDI and IPDI may have respiratory sensitization potential in humans and TMI and TMXDI do not. Following dermal exposure of BALB/c mice, all six isocyanates induced cytokines characteristic of a Th2 response. Although LLNAs suggested that the doses chosen for the RPA were immunologically equivalent, the isocyanates tested differentiated into two groups, high responders and low responders. However, two of the low responders (TMI and TMXDI) were further tested and induced higher levels of Th2 cytokine message than dinitrochlorobenzene (not an asthmagen). Further study of these chemicals is needed to determine whether the Th2 cytokine responses observed for these low responders is predictive of asthmagenic potential or represents an insufficient signal.

Phenyl isocyanate is a potent chemical sensitizer

Diisocyanates, as a chemical class, are recognized for their ability to cause respiratory and dermal sensitization. By contrast, monofunctional isocyanates have not been associated with cases of clinical sensitization. We investigated the immunologic activity of phenyl isocyanate (PI), an aromatic monoisocyanate, which is a trace constituent in commercial diphenyl methane diisocyanate (MDI) products, since it is has been reported to cause an "asthma-like' syndrome in rats. The potency of PI as a contact sensitizer was assessed using the mouse ear swelling test. PI was found to be the most potent isocyanate tested yielding an SD50 (dose predicted to sensitize 50% of the mice) of 0.04 mumol/kg, compared with SD50 values of 0.5, 2.1, and 30.4 mumol/kg, for the diisocyanate sensitizers hexamethylene diisocyanate, MDI, and toluene diisocyanate (TDI), respectively. When tested for ability to stimulate humoral immune responses, antibody titers to PI were more than ten-fold greater than those induced by TDI. The mean hapten-specific IgG antibody titer to PI was 1.4 x 10(4), compared with 1.3 x 10(3) for TDI. The anti-PI IgG1 anaphylactic antibody titer (1.2 x 10(4) was significantly greater than the anti-TDI IgG1 titer of 6.4 x 10(2). Hapten-specific IgE was not detected to either isocyanate. These results indicate that phenyl isocyanate is a potent inducer of both cellular and humoral immune responses. This activity may be a significant contribution to the sensitization potential of commercial products in which it is a trace contaminant.

Relations between in vitro cytotoxicity and crosslinked dermal sheep collagens

Collagen-based biomaterials have found various applications in the biomedical field. However, collagen-based biomaterials may induce cytotoxic effects. This study evaluated possible cytotoxic effects of (crosslinked) dermal sheep collagen (DSC) using a 7-d-methylcellulose cell culture with human skin fibroblasts. Non-crosslinked DSC (NDSC), hexamethylene-diisocyanate-crosslinked DSC (HDSC), and glutaraldehyde-crosslinked DSC (GDSC), their extracts (1 x 10 d to 4 x 10 d extracts), or the corresponding extracted DSC samples were tested. Cell growth was evaluated by cell counting, while cell morphology was assessed by light microscopy and transmission-electron microscopy. Both GDSC and, to a lesser extent, HDSC, induced cytotoxicity, observed as inhibited cell growth and deviant cell morphology. The deviant morphology consisted of extensive accumulations of lipid, reduction in the amount and dilatation of rough endoplasmatic reticulum, increased inclusions of cell remnants, and relatively rounded cell membranes. With HDSC, both primary cytotoxicity, due to extractable products from the material, and secondary cytotoxicity, possibly due to a release of cytotoxic products resulting from enzymatic cell-biomaterial interactions, could be discriminated. With GDSC, however, no clear distinction between primary and secondary cytotoxicity could be made. With NDSC, only primary cytotoxicity, measured as low inhibition of cell proliferation, but without deviant morphology, was observed. These remarkable differences in cytotoxicity are discussed in relation to residual agents and specific crosslinks present in DSCs as a consequence of processing and the crosslinking agents used. The residual agents and the specific crosslinks give rise to differences in direct release of products and in sensitivity to hydrolysis and enzymatic breakdown.

In vivo degradation of processed dermal sheep collagen evaluated with transmission electron microscopy

The in vivo degradation of hexamethylenediisocyanate-tanned dermal sheep collagen was studied with transmission electron microscopy. Discs of hexamethylenediisocyanate-tanned dermal sheep collagen were subcutaneously implanted in rats. Both an intra- and an extracellular route of degradation could be distinguished. In addition to normal components of a typical foreign body reaction, remarkable phenomena, such as locally deviant neutrophil morphology, infiltration of basophil-like cells, indications of foreign body multinucleate giant cells formed from different cell types, aluminium silicate accumulations and calcium phosphate depositions, were observed. Foreign body multinucleate giant cells intracellularly degraded hexamethylenediisocyanate-tanned dermal sheep collagen after internalization. Both internalized and cellularly enveloped hexamethylenediisocyanate-tanned dermal sheep collagen degraded by the detachment of fibrils. Another extracellular route of degradation was characterized by calcium phosphate depositions in large bundles of hexamethylenediisocyanate-tanned dermal sheep collagen. From 6 wk, the hexamethylenediisocyanate-tanned dermal sheep collagen implant was replaced by rat connective tissue, which was subsequently also degraded. After 15 wk, the presence of basophil-like foreign body multinucleated giant cells containing aluminium/silicon-crystalline accumulations still persisted. These phenomena were related to the specific nature of the material used and suggest cytotoxicity. They emphasize the need for detailed evaluation at the ultrastructural level of newly developed biomaterials before they can be used for medical applications.

Nitrocellulose-based RAST to detect IgE antibodies in workers hypersensitive to diphenylmethane-4,4'-diisocyanate

RAST was performed using both nitrocellulose (NC) and paper discs to compare these solid supports for ability to detect IgE antibodies in sera from two patients with hypersensitivity to the industrial chemical, diphenylmethane-4,4'-diisocyanate (MDI). NC disc had distinct advantages in ease of coupling the hapten-conjugate antigen to discs, and in use of lesser amounts of antigen. With regard to sensitivity, NC discs were better able to distinguish the positive sera from 34 control sera including those with elevated levels of total IgE. In both RAST systems, inhibition studies indicated high specificity of antibodies for MDI, with no reactivity detected toward the serum albumin portion of the MDI-HSA conjugate antigen. However, only in the NC RAST did antibodies from both patients react with p-tolyl isocyanate inhibitor. Based on the above results, NC discs displayed several advantages in RAST and are recommended for routine serological assay of isocyanate-specific antibodies.

Respiratory effects of inhaled isocyanates

Numerous industrial chemicals are known to cause allergic reactions in the lung. Prominent among such chemicals are isocyanates, the starting material in the production of polyurethanes. In view of the extensive worldwide production of isocyanates (1.5 million metric tons per year), and the diversity of products manufactured from isocyanates (i.e., cars, airplanes, furniture, bedding, etc.) there is considerable potential for adverse health effects associated with exposure to isocyanates. Syndromes of immediate respiratory reactivity, delayed-onset sensitivity, and hypersensitivity pneumonitis have all been associated with isocyanate exposure. However, little is known concerning how sensitivity develops, which individuals are most likely to become sensitized, or how to best detect early sensitivity. Answers to such questions are beginning to emerge from the recent development of animal models of lung sensitivity. These models will be discussed together with their application to clinical situations.