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Journal of Virology, March 2003, p. 3866-3870, Vol. 77, No. 6
0022-538X/03/$08.00+0 DOI: 10.1128/JVI.77.6.3866-3870.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
The Type B Leukemogenic Virus Truncated Superantigen Is Dispensable for T-Cell Lymphomagenesis
Farah Mustafa,1,
Sanchita Bhadra,1 Dennis Johnston,2 Mary Lozano,1 and Jaquelin P. Dudley1*
Section of Molecular Genetics and Microbiology and Institute for Cellular and Molecular Biology, the University of Texas at Austin, Austin, Texas 78712,1
Department of Biomathematics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 770302
Received 9 October 2002/
Accepted 20 December 2002

ABSTRACT
Type B leukemogenic virus (TBLV) is a variant of mouse mammary
tumor virus (MMTV) that causes T-cell lymphomas in mice. We
have constructed a TBLV-MMTV hybrid, pHYB-TBLV, in which 756
bp of the C3H MMTV long terminal repeat (LTR) was replaced with
438 bp of the TBLV LTR. Intraperitoneal injection of pHYB-TBLV
transfectants consistently resulted in T-cell lymphomas in 50%
of injected weanling BALB/c mice with an average latency period
of 5.7 (± 1.5) months. Transfectants of pHYB-TBLV containing
a double-frameshift mutation in the truncated superantigen gene
(
sag) induced T-cell lymphomas with similar incidences, latency
periods, and phenotypes, suggesting that
cis-acting elements
in the TBLV LTR determine disease specificity.

TEXT
Mouse mammary tumor virus (MMTV) is a slow-acting oncogenic
retrovirus that causes mammary tumors in mice after a long period
(6 to 9 months) (
1,
29). An MMTV variant, type B leukemogenic
virus (TBLV), has been demonstrated to cause thymic lymphomas
with a short latency period (1.5 to 2 months) when injected
intrathymically into neonatal mice (
2). Compared to MMTV, TBLV
has a deletion of 443 nucleotides and a substitution of 124
nucleotides in the U3 region of the long terminal repeat (LTR)
(
3). This deletion removes a negative regulatory element (NRE)
present in the MMTV LTR (
17-
19,
23,
36,
37), which has been
implicated in tissue-specific MMTV expression (
6,
30). The substitution
creates a triplication of a 62-bp element comprised of sequences
flanking the deletion (
3) (Fig.
1A).
The LTR deletion and substitution also truncate the open reading
frame (ORF) encompassing the U3 region of the LTR (Fig.
1A).
This ORF encodes a 37-kDa, type II transmembrane glycoprotein
known as superantigen (Sag) that functions in the milk-borne
transmission of MMTV (
1,
7,
9,
10,
16) by the stimulation of
T cells through the T-cell receptor (TCR) (
33,
35). The truncated
Sag protein encoded in the TBLV LTR lacks the entire C-terminal
region that interacts with the TCR, yet it retains the transmembrane
region, a part of the extracellular region that includes the
five glycosylation sites, a single proteolytic processing site,
and the major histocompatibility complex class II protein binding
motif (
12,
20). This study shows that the truncated TBLV Sag
protein is dispensable for virally induced T-cell lymphomas.
A 438-bp region of the TBLV LTR alters MMTV disease specificity.
Earlier studies showed that the tropism of the MMTV hybrid provirus pHYB-MTV (31) can be altered by replacement of the 3' LTR with that from thymotropic MMTVs (34). However, these studies did not distinguish between the activity of the truncated Sag protein and that of the cis-acting elements in the LTRs. Therefore, TBLV-specific sequences from the ClaI-to-SstI region in the LTR (438 bp), including the T-cell-specific enhancer (21), were obtained by PCR and used to replace 756 bp from the ClaI-to-SstI region of the pHYB-MTV 3' LTR. Subsequently, Jurkat T cells stably transfected with pHYB-TBLV were derived with the assumption that these cells would express high levels of HYB-TBLV, thus allowing high-efficiency infection and tumorigenesis in mice. Jurkat cells were also transfected with pHYB-TBLV that had been modified by the insertion of two frameshift mutations in the truncated sag gene (pHYB-TBLVsagDFS). Transfected cells were then tested for expression of the Gag CA protein by Western blotting. Comparable amounts of viral proteins from the two constructs were detectable in cell lysates (Fig. 1B) and in culture supernatants (Fig. 1C). Since the frameshift mutations also lie within the transcriptional regulatory sequence, we inserted the 3' LTR from pHYB-TBLVsagDFS and pHYB-TBLV into a luciferase expression vector (32) for transient-transfection experiments in Jurkat cells as described previously (21). No significant difference between the luciferase activities of the two LTRs was observed (data not shown).
Subsequently, 2 x 107 Jurkat cells stably expressing HYB-TBLV or HYB-TBLVsagDFS were injected intraperitoneally into weanling BALB/cJ mice. Injection of HYB-TBLV transfectants resulted in tumors in 50% of the mice with a latency period of 5.7 (± 1.5) months (mean [± standard deviation]), whereas mice receiving HYB-TBLVsagDFS transfectants developed T-cell lymphomas with a 60% incidence and an average latency period of 4.8 (± 0.6) months (Table 1) (P > 0.05). Expression of HYB-TBLV or the sag frameshift mutant in these tumors was confirmed by reverse transcription-PCR and sequencing as described by Mustafa et al. (26) (data not shown). These results revealed that an
440-bp region from the TBLV LTR was sufficient to alter MMTV disease tropism and that the truncated TBLV Sag protein was not essential for the development of virally induced tumors.
View this table:
[in this window]
[in a new window]
|
TABLE 1. Incidence and latency of tumors induced by clonal TBLV proviruses after injection of transfected Jurkat T cells
|
Similar clonalities and phenotypes of HYB-TBLV and HYB-TBLVsagDFS-induced T-cell lymphomas.
To determine whether the phenotypes of T-cell lymphomas induced
by the wild-type (HYB-TBLV) and the mutated (HYB-TBLV
sagDFS)
viruses were similar, we analyzed the tumor cell populations
by flow cytometry (Table
2). Most tumors had a high percentage
of Thy1.2
+ cells, thereby confirming the T-cell origin of the
cells. The Thy1.2
+ cell populations were heterogeneous with
respect to the surface distribution of CD4 and CD8 markers,
but this distribution was not significantly different for the
two groups. The tumors were not outgrowths of the originally
injected cells since the antibodies used for cell surface staining
did not react with Jurkat cells.
View this table:
[in this window]
[in a new window]
|
TABLE 2. Comparison of cell surface markers on lymphomas induced by HYB-TBLV and HYB-TBLVsagDFS in adult BALB/c micea
|
To analyze the clonality of thymic lymphomas induced by the
wild-type and
sag frameshift TBLV strains, TCR ß and

chain rearrangements were analyzed by Southern blotting (Fig.
2). Most of the tumors induced by either virus showed rearrangement
of both TCR chains, and many tumors had significant clonal populations.
Therefore, the tumors induced by the wild-type and mutated TBLV
hybrid proviruses were very similar and comparable to tumors
induced by intrathymic injection of TBLV virions into newborn
mice (
8,
24,
25). These results, combined with cell surface
analysis, suggest that the tumors were oligoclonal.
Lack of requirement for the truncated TBLV Sag during tumorigenesis.
Intrathymic injection with tissue culture supernatant from a
TBLV-induced T-cell lymphoma line, 485-10, produces T-cell lymphomas
in 90% of injected animals (
2,
4,
5,
28). The disadvantages
of this method are that these virus preparations are uncloned
and heterogeneous (our unpublished data). Our clonal TBLV preparations
were not injected into newborn mice, but the lower incidence
and longer latency period of TBLV-induced tumors in the present
study might be explained in two ways. First, since the thymus
starts to regress in adult mice, less thymic tissue is available
for TBLV infection and random insertions that lead to tumors
(
8,
27). Second, the route of infection may influence the efficiency
of tumorigenesis. In previous experiments, culture supernatants
were used to introduce TBLV intrathymically, thus directly inoculating
the target for oncogenesis, whereas TBLV-expressing Jurkat T
cells were introduced into the mouse peritoneum and must have
been transmitted to thymocytes by an unknown mechanism. Inoculations
of Jurkat cell transfectants into newborn mice may provide more
efficient cell-to-cell transmission than injections of virions,
which have given erratic results (unpublished observations).
The truncated Sag protein was not required for virally induced T-cell lymphomas in adult BALB/c mice when transfected Jurkat cells were used for TBLV infection (Table 2). This result may be specific for this route of infection, or, alternatively, the Sag proteins expressed from the endogenous MMTVs in BALB/c mice may complement the sag-null virus. However, it appears likely that the truncated Sag protein has no role in TBLV-induced tumors and that the loss of the NRE and/or other cis-acting elements is responsible for altering the viral disease specificity (14). The development of a cloned TBLV provirus provides us with a valuable tool for molecular dissection of the elements required for changes in MMTV disease specificity.

ACKNOWLEDGMENTS
This work was supported by R01 grants CA34780 and CA77760 from
the National Institutes of Health.
We thank members of the Dudley lab for useful comments on the manuscript.

FOOTNOTES
* Corresponding author. Mailing address: Section of Molecular Genetics and Microbiology and Institute for Cellular and Molecular Biology, The University of Texas at Austin, 100 W. 24th St., Austin, TX 78705. Phone: (512) 471-8415. Fax: (512) 471-7088. E-mail:
jdudley{at}uts.cc.utexas.edu.

Present address: Department of Medical Microbiology, Faculty of Medicine and Health Sciences, The United Arab Emirates (UAE) University, Al Ain, United Arab Emirates. 

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Journal of Virology, March 2003, p. 3866-3870, Vol. 77, No. 6
0022-538X/03/$08.00+0 DOI: 10.1128/JVI.77.6.3866-3870.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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