Immunosuppressive Effect of Subchronic Exposure to a Mixture of Eight Heavy Metals, Found as Groundwater Contaminants in Different Areas of India, Through Drinking Water in Male Rats
Immunotoxicity is an important health hazard of heavy metal exposure. Because the risk of combined exposure in the population cannot be neglected, we examined whether subchronic exposure to a mixture of metals (arsenic, cadmium, lead, mercury, chromium, nickel, manganese, and iron) via drinking water at contemporary Indian groundwater contamination levels and at concentrations equivalent to the WHO maximum permissible limit (MPL) in drinking water can induce immunotoxicity in male rats. Data on groundwater contamination with metals in India were collected from literature and metals were selected on the basis of their frequency of occurrence and contamination level above the MPL.
Male albino Wistar rats were exposed to the mixture at 0, 1, 10, and 100 times the mode concentrations (the most frequently occurring concentration) of the individual metals in drinking water for 90 days. In addition, one group was exposed to the mixture at a concentration equal to the MPL of the individual metal and another group was used as positive control for immune response studies. The end points assessed were weights of organs, hematological indices, humoral and cell-mediated immune responses, and histopathology of skin and spleen. The MPL and 1× doses did not significantly affect any of the parameters and none of the doses induced any significant changes after 30 days of exposure.
The mixture at 10× and 100× doses increased the relative weight of the spleen, but that of thymus, adrenals, and popliteal lymphnodes were increased with the 100× dose. After 90 days, 10× and 100× doses decreased serum protein and globulin contents and increased the albumin:globulin ratio; the albumin level was decreased only with the 100× dose.
After 60 days, the total erythrocyte count (TEC), hemoglobin (Hb) level, and packed cell volume (PCV) were decreased with the 100× dose, whereas after 90 days, 10× and 100× doses reduced the TEC, total leukocyte count, Hb level, PCV, mean corpuscular volume, and mean corpuscular hemoglobin. With the 100× dose, the lymphocyte count was decreased after 60 and 90 days, but the neutrophil number was increased after 90 days.
Antibody titer was decreased after 75 days with the 100× dose, but after 90 days, it was decreased with both the 10× and 100× doses. In delayed-type hypersensitivity response, these two doses decreased ear thickness after 24 and 48 h and skin biopsies showed a dose-dependent decrease in inflammatory changes. Histologically, the spleen revealed depletion of lymphoid cells and atrophic follicles with reduced follicular activity with higher doses.
The findings suggest that hematopoietic and immune systems are toxicologically sensitive to the mixture, which could lead to anemia and suppression of humoral and cell-mediated immune responses in male rats at 10 and 100 times the mode concentrations of the individual components in contaminated groundwater