Arenavirus Infection: Lymphocytic Choriomeningitis Virus Infection

Mus musculus is the natural reservoir host for lympho­cytic choriomeningitis virus (LCMV), and mice have carried this virus throughout the world from their origi­nal Old World niche.

Epizootiology and Pathogenesis

LCMV is not ubiquitous, in that isolated mouse popula­tions may or may not be enzootically infected. It is fortunately rare among contemporary laboratory mouse

populations, but may be a contaminant of biological products derived from mice. LCMV has also been found in pet mice and colonies of mice raised for feeding other species, including non-human primates.

Enzootic infection in a mouse population is main­tained by vertical transmission from dam to fetus and neonates. Infection of the fetus occurs during early pregnancy, and ova can be infected prior to implanta­tion. There is no evidence for transmission by coitus. Nearly every cell in the fetus may become infected non- cytolytically, with no significant adverse effect, although reduced litter sizes and runted pups may occur. The widely disseminated fetal infection involves the immature thymus, resulting in selective immune toler­ance, with negative selection and depletion of LCMV- responsive CD8 T cells. This state of tolerance is highly LCMV selective, with otherwise normal immune respon­siveness to other antigens. A similar scenario can occur if pups are infected as neonates. The state of LCMV immune tolerance allows multisystemic, persistent, sub- clinical infection, with the mice growing to reproductive maturity and perpetuating the cycle to the next genera­tion. Immune-tolerant adult females that were infected as neonates can not only infect their own young in utero but also do not confer passive immunity to neonates, which further facilitates spread of the virus due to com­munal nursing behavior. Tolerance is not absolute, as mice develop LCMV-specific antibody, but antibody is non-neutralizing and complexed with excess viral anti­gen, which tends to be deposited in tissues, including arterial walls, choroid plexus, and glomeruli. Eventually, tolerance breaks down further, resulting in chronic lym­phocytic infiltrates in multiple tissues and immune complex glomerulonephritis (late disease).

In contrast, natural or experimental infection of adult, immunocompetent mice follows a distinctly dif­ferent course. Experimental infection results in a wide variety of disease manifestations, depending upon host and virus factors. Following natural exposure, or natural routes of experimental inoculation (intranasal or oral), immunocompetent adult mice typically develop short­term, acute infections from which they recover and seroconvert. However, parenteral inoculation with “aggressive” strains of virus results in disseminated infec­tion of multiple organs, followed by a host immune response with CD8 T-cell-mediated disease. When virus is inoculated intracerebrally, disease is characterized by immune-mediated lymphocytic choriomeningitis (espe­cially with “neurotropic” strains), whereas intra­peritoneal inoculation results in immune-mediated hepatitis. In contrast, inoculation of mice with high doses of virus that are “docile” and “viscerotropic” results in immune exhaustion (in contrast to immune tolerance) and therefore mice develop no clinical, T-cell- mediated disease (thus, the term “docile”). The mecha­nism for immune exhaustion is selective targeting and high affinity for alpha-dystroglycan receptors on den­dritic cells. The virus thus initially targets dendritic cells in the marginal zones of the spleen and lymph nodes, and then spreads to T-cell regions, with subsequent T-cell-mediated immune destruction of infected lymph­oid tissue.

LCMV strains and isolates cannot be differentiated serologically, as LCMV is a monotypic quasispecies. Nevertheless, there are a number of clonal laboratory strains with differing experimental tissue tropism and biological behavior, including Armstrong, Traub, WE, Pasteur, and others. Experimental disease is dependent upon virus strain, dose, route of inoculation, and host factors, including age, strain, and immunocompetence. A significant determinant of susceptibility to the adult, immune-mediated form of experimental disease is linked to the H-2 locus. Mice with H-2q∕q (e.g., SWR) or H-2q∕k (e.g., C3H.Q) haplotypes are disease suscep­tible, whereas mice of the H-2k∕k haplotype (e.g., C3H/ He) are disease resistant. H-2 haplotype is associated with CD8 T-cell responsiveness, but CD4 T cells, B cells, NK cells, and interferons, among other factors, contrib­ute to LCMV immunity. The adult form of infection following parenteral inoculation is largely an experi­mental phenomenon, but it provides insight into the outcome of inoculation of mice with contami­nated biological material. LCMV is a frequent contam­inant of transplantable tumors. Considering the wide variety of immune-deficient mice being utilized today, and the many immune factors that are deter­minants of controlling outcome of infection or dis­ease, awareness of the full spectrum of LCMV biology is useful.

FIG. 1.24. Lymphocytic infiltration of meninges of a mouse inoculated intracerebrally with lymphocytic choriomeningitis virus (LCMV).

Pathology

Clinical signs of natural LCMV infection are minimal, but can include runting in infant mice, reduced litter size, and chronic wasting in older mice if infection is occurring in utero within the colony. Microscopic lesions are likewise nonspecific and most likely to be found in persistently infected aged mice, which may develop vasculitis, glomerulonephritis, and lymphocytic infiltration in multiple tissues, including brain, liver, adrenal, kidney, and lung. Acute disease is largely an experimental phenomenon arising from parenteral inoculation with high doses of virus. Infection with “aggressive” strains of virus results in generalized lymph­adenopathy with lymphoid hyperplasia, lymphocytic infiltrates in multiple tissues, necrotizing hepatitis, and lymphocytic choriomeningitis (especially if inocu­lated intracerebrally) (Fig. 1.24). Infection with “docile” strains of virus results in severe generalized lymphoid depletion in T-cell regions of the thymus, spleen, and lymph nodes. Although natural infection of nude mice has been documented, pathology was not described.

Diagnosis

Definitive diagnosis of LCMV infection cannot be based on pathology. Serology with recombinant nucleoprotein antigen has circumvented the hazards of growing the virus for antigen. However, serology may be problem­atic, since horizontal infection among adult mice is inefficient and likely to cause seroconversion among a very few mice within a population. Mice infected in utero or as neonates are immune-tolerant to LCMV and do not efficiently seroconvert, or circulating anti­body is complexed with antigen. Adult mice infected with docile types of virus may have immune exhaustion, and not seroconvert. Thus, serological testing must be applied to a large sample size and can be enhanced by cohousing adult sentinel mice with mice suspected to be persistently infected. LCMV can be confirmed in suspect tissues with a variety of bioassay approaches, such as

MAP testing, but this has been supplanted by PCR. Differential diagnoses for runting in infant mice include a number of other viral infections. Chronic illness in older mice must be differentiated from generalized lym­phoproliferative disorders, amyloidosis, glomerulo­nephritis, and chronic renal disease of aging mice.