Extreme susceptibility of African naked mole rats (Heterocephalus glaber) to experimental infection with herpes simplex virus type 1

Abstract

Herpes simplex virus type 1 (HSV1) is widely used as a gene delivery vector in a variety of laboratory animals. In a recent study, a thymidine-kinase-inactive (replication-conditional) HSV1 used as a delivery vector was lethal in naked mole rats, whereas mice infected with the identical virus showed no adverse effects. This result prompted us to undertake a controlled comparative histologic study of the effect of HSV1 infection on naked mole rats and mice. Replication-competent and replication-conditional HSV1 caused widespread inflammation and necrosis in multiple organ systems of naked mole rats but not mice; naked mole rats infected with replication-defective virus showed no adverse effects. We conclude that the lethality of HSV1 for naked mole rats is likely the result of overwhelming infection, possibly in part due to this species' natural lack of proinflammatory neuropeptides at the initial site of infection.

Figure 1.

The creation of replication-conditional, replication-competent, and replication-defective viruses. For replication-conditional virus, an expression cassette was inserted into the HSV1 thymidine kinase [TK(UL23)] gene of HSV1 strain KOS. Insertion of the expression cassette into the thymidine kinase gene prevents expression of the enzyme and renders the virus incapable of replication in nondividing cells, such as neurons. For replication-competent virus, an expression cassette was inserted HSV1 strain KOS between the UL36 and UL37 genes. This virus was capable of replication in all tissues but was considered to have low virulence. For replication-defective virus, an expression cassette was inserted into the ICP4⁻ mutant of HSV1 between the UL36 and UL37 genes. This virus was incapable of replication.

Figure 2.

Photomicrographs of a naked mole rat that received replication-competent HSV1. (A) Large numbers of ground-glass (black arrow) and small numbers of Cowdry type A (double black arrow) intranuclear inclusion bodies in the liver. Hematoxylin and eosin stain; bar, 100 μm. (B) Large numbers of immunoperoxidase-positive hepatocytes. Note the intracytoplasmic and intranuclear staining of the cells. Avidin–horseradish peroxidase stain with hematoxylin counterstain; bar, 100 μm. (C) Acute splenitis with severe follicular degeneration and necrosis (black arrow). Hematoxylin and eosin stain; bar, 200 μm. (D) Note the diffuse immunoperoxidase-positive staining of lymphocytes and mononuclear cells. Avidin–horseradish peroxidase with hematoxylin counter stain; bar, 200 μm. (E) Infiltration of the villous lamina propria with medium numbers of neutrophils (black arrow). Note the Cowdry type A intranuclear inclusion body in a mononuclear cell (double black arrow). Hematoxylin and eosin stain; bar, 50 μm. (F) Moderate necrosis of gut-associated lymphoid tissue and mononuclear cells within the villous lamina propria of the small intestine. Hematoxylin and eosin stain; bar, 200 μm.

Figure 3.

Photomicrographs of a NMR that received replication-conditional HSV1. (A) Focal neutrophilic infiltration (black arrow) with mild diffuse vacuolation and increased granularity of the hepatocytes. Note the absence of intranuclear inclusion bodies. Hematoxylin and eosin stain; bar, 100 μm. (B) Focal immunoperoxidase-positive staining hepatocytes. Avidin–horseradish peroxidase stain with hematoxylin counterstain; bar, 100 μm. (C) Small splenic follicle with no distinct germinal center and decreased lymphocytes in the mantel and marginal zones. Note the extramedullary hematopoiesis in the red pulp surrounding the follicle (black arrows). Hematoxylin and eosin stain; bar, 200 μm. (D) Note the immunoperoxidase staining of lymphocytes and mononuclear cells within the splenic follicle. Avidin–horseradish peroxidase with hematoxylin counter stain; bar, 200 μm. (E) Focal acute enteritis with mild to moderate suppurative necrosis of gut-associated lymphoid tissue in the small intestine. Hematoxylin and eosin stain; bar, 200 μm. (F) Photomicrograph of the small intestine of a naked mole rat that received replication-defective HSV1. Note the normal gut-associated lymphoid tissue in the small intestine with a germinal center (black arrow) and thick mantle of small lymphocytes (double black arrow). Hematoxylin and eosin stain; bar, 200 μm.