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Infection and Immunity, August 2000, p. 4462-4469, Vol. 68, No. 8
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Antiviral Activity of Shiga Toxin 1: Suppression of Bovine
Leukemia Virus-Related Spontaneous Lymphocyte Proliferation
Witold A.
Ferens and
Carolyn J.
Hovde*
Department of Microbiology, Molecular Biology
and Biochemistry, University of Idaho, Moscow, Idaho 83844
Received 14 March 2000/Accepted 1 May 2000
Human infections with Shiga toxin (Stx)-producing Escherichia
coli (STEC) cause hemorrhagic colitis. The Stxs belong to a large
family of ribosome-inactivating proteins (RIPs) that are found in a
variety of higher plants and some bacteria. Many RIPs have potent
antiviral activity for the plants that synthesize them. STEC strains,
both virulent and nonvirulent to humans, are frequently isolated from
healthy cattle. Interestingly, despite intensive investigations, it is
not known why cattle carry STEC. We tested the hypothesis that Stx has
antiviral properties for bovine viruses by assessing the impact of Stx
type 1 (Stx1) on bovine peripheral blood mononuclear cells (PBMC) from
cows infected with bovine leukemia virus (BLV). PBMC from BLV-positive
animals invariably displayed spontaneous lymphocyte proliferation (SLP) in vitro. Stx1 or the toxin A subunit (Stx1A) strongly inhibited SLP.
Toxin only weakly reduced the pokeweed mitogen- or
interleukin-2-induced proliferation of PBMC from normal (BLV-negative)
cows and had no effect on concanavalin A-induced proliferation.
The toxin activity in PBMC from BLV-positive cattle was selective for
viral SLP and did not abrogate cell response to pokeweed mitogen- or
interleukin-2-induced proliferation. Antibody to virus or Stx1A was
most effective at inhibiting SLP if administered at the start of cell
culture, indicating that both reagents likely interfere with
BLV-dependent initiation of SLP. Stx1A inhibited expression of BLV p24
protein by PBMC. A well-defined mutant Stx1A (E167D) that has decreased
catalytic activity was not effective at inhibiting SLP, suggesting the
inhibition of protein synthesis is likely the mechanism of toxin
antiviral activity. Our data suggest that Stx has potent antiviral
activity and may serve an important role in BLV-infected cattle by
inhibiting BLV replication and thus slowing the progression of
infection to its malignant end stage.
*
Corresponding author. Mailing address: Department of
Microbiology, Molecular Biology and Biochemistry, University of Idaho, Moscow, ID 83844. Phone: (208) 885-5906. Fax: (208) 885-6518. E-mail:
cbohach{at}uidaho.edu.

Publication no. 00529 of the Idaho Agriculture Experiment
Station.
Infection and Immunity, August 2000, p. 4462-4469, Vol. 68, No. 8
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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