In vivo cytokine response to experimental feline infectious peritonitis virus infection

Abstract

Feline infectious peritonitis virus (FIPV) is a coronavirus that causes sporadic fatal disease in cats characterized by vasculitis, granulomatous inflammation and effusive pleuritis/peritonitis. Histologic changes in lymphoid tissues include lymphoid hyperplasia, lymphoid depletion, histiocytosis, and granuloma formation. Although viremia occurs, histologic lesions are not found uniformly throughout lymphoid tissues. We used experimental infection of cats with a highly pathogenic FIPV isolate, UCD8, to study histologic lesions, virus replication, and cytokine expression in multiple lymphoid tissues during the effusive phase of disease. Viral RNA was found in 76% of central tissues (mediastinal lymph node, spleen, mesenteric lymph node) examined, as compared to 27% of peripheral tissues (popliteal lymph node, cervical lymph node, femoral bone marrow). All tissues positive for virus replication also demonstrated lymphoid depletion. Generally, affected tissues had lower levels of IL-4 and IL-12-p40 mRNA and higher levels of IL-10 mRNA. Although no differences in IFN-gamma or TNF-alpha mRNA were measured, TNF-alpha protein expression was greater in affected tissues and demonstrated a shift in the source of TNF-alpha from macrophages to lymphocytes. Together, these results colocalize FIPV replication, lymphocyte depletion in tissues, and alterations in cytokine transcription and translation. A possible role for TNF-alpha in the previously described FIPV-induced lymphocyte apoptosis is also suggested.

Fig. 1

FIPV-infected cats develop a marked lymphopenia. White blood cell counts (WBC), neutrophil counts (Neuts) and lymphocyte counts (Lymphs) are shown at pre-infection (pre) and terminal (term) time points. The box represents the median 75th percentile (horizontal line is median) and the ‘whiskers’ show the upper and lower 12.5th percentile. ^∗P=0.003, significantly different from WBC-pre; ^∗∗P<0.0001, significantly different from Lymphs-pre.

Fig. 2

FIPV associated changes in cytokine mRNA expression. Cytokine transcription levels in FIPV positive (N=28) and negative (N=27) central lymphoid tissues (mesenteric lymph node, mediastinal lymph node, spleen) were quantified by competitive RT-PCR. Results are shown as the number of cytokine mRNA molecules/μg of total RNA (Y-axis). Bars represent standard error of the mean.

Fig. 3

IL-10:IL-12 ratio in FIPV positive and negative tissues. IL-10:IL-12 ratios were markedly higher in most FIPV positive tissues.

Fig. 4

Localization of FIPV antigen, IL-10, and TNF-α in mediastinal lymph nodes. (A) FIPV spike-protein was observed within macrophages scattered throughout the lymph node (magnification, 100×). (B) IL-10 positive macrophages in a FIPV positive node (magnification, 200×). (C) TNF-α expressing macrophages (wide arrow) and lymphocytes (narrow arrow) in a FIPV negative node. (D) A cellular follicle stained with hematoxylin–eosin in a FIPV positive node with areas of necrosis/apoptosis is shown in (E) to have high levels of TNF-α production from lymphocytes (magnification, 100×). (F) A lymphocyte-depleted follicle stained with hematoxylin–eosin has many scattered FIPV spike antigen positive cells (G) and TNF-α expressing lymphocytes (H) (magnification, 100×).