Lassa virus infection in experimentally infected marmosets: liver pathology and immunophenotypic alterations in target tissues

Abstract

Lassa virus causes thousands of deaths annually in western Africa and is considered a potential biological weapon. In an attempt to develop a small nonhuman primate model of Lassa fever, common marmosets were subcutaneously inoculated with Lassa virus strain Josiah. This inoculation resulted in a systemic disease with clinical and morphological features mirroring those in fatal human Lassa infection: fever, weight loss, high viremia and viral RNA load in tissues, elevated liver enzymes, and severe morbidity between days 15 and 20. The most prominent histopathology findings included multifocal hepatic necrosis with mild inflammation and hepatocyte proliferation, lymphoid depletion, and interstitial nephritis. Cellular aggregates in regions of hepatocellular necrosis were largely composed of HAM56-positive macrophages, devoid of CD3-positive and CD20-positive cells, and characterized by marked reductions in the intensity of HLA-DP, DQ, DR staining. A marked reduction in the major histocompatibility complex class II expression was also observed in the lymph nodes. Immunophenotypic alterations in spleen included reductions in overall numbers of CD20-positive and CD3-positive cells and the disruption of lymphoid follicular architecture. These findings identify the common marmoset as an appropriate model of human Lassa fever and present the first experimental evidence that replication of Lassa virus in tissues is associated with alterations that would be expected to impair adaptive immunity.

FIG. 1.

Lassa virus infection in common marmosets. (A) alanine aminotransferase (ALT) and (B) albumin (ALB) levels in plasma of four monkeys infected with Lassa virus. (C) Gross pathology indicates an enlarged liver with pale foci in animal 17280, which was infected with 1 × 10⁶ PFU of Lassa virus. (D) Viral load in tissues of four infected monkeys was measured by qRT-PCR as described in Materials and Methods and was expressed as means + standard deviations. Error bars indicate standard deviations. LN, lymph nodes.

FIG. 2.

Hepatic pathology in Lassa-inoculated common marmosets. Multifocal random hepatic necrosis accompanied by a mixed inflammatory cell infiltrate consisting of macrophages and lymphocytes (A). Degenerate hepatocytes contained well-circumscribed eosinophilic to amphophilic cytoplasmic inclusions, and individual hepatocyte necrosis was apparent (inset, Councilman body) (B). Cellular aggregates in regions of hepatocellular necrosis were largely composed of HAM56 macrophages and devoid of (C) CD3-positive and (D) CD20-positive cells. A marked reduction in the intensity of HLA-DP, DQ, DR staining (E) was observed relative to normal control tissue (inset). Increased numbers of cells positive for the proliferation marker Ki67/MIB1 were observed (F).

FIG. 3.

Pulmonary pathology in common marmosets infected with Lassa virus. A multifocal interstitial pneumonitis was observed in two of four animals (Table 1), as evidenced by septal thickening, alveolar edema, and increased numbers of inflammatory cells (A). In these animals, multifocal periarterial edema (B) and multifocal hypertrophy of pleural mesothelial cells (C) were observed.

FIG. 4.

Immunophenotypic alterations in lymph node during experimental Lassa infection. A marked reduction in the intensity of HLA-DR staining was observed in lymph nodes obtained from Lassa-infected animals (B) compared to that seen in lymph nodes from controls (A). Increased numbers of HAM56-positive cells were noted in the infected animals (D) compared with controls (C), and the discrete staining observed in normal lymph node was largely absent.

FIG. 5.

Immunophenotypic alterations in spleen of marmosets infected with Lassa virus. Overall numbers of CD20-positive B lymphocytes were reduced in Lassa virus-infected animals (B) compared with that in controls (A), and lymphoid follicles lacked their normal architecture. Similarly, CD3-positive T cells within the periarteriolar sheath were reduced in number or absent from the infected animals (D) compared to controls (C).