We next assessed the binding affinity of the p11C and p41A peptides for
Mamu-A*01, utilizing a quantitative in vitro binding assay
(8). This assay utilized 2 × 106
C1R-Mamu-A*01 transfectants to measure the inhibition of binding of
105 cpm of iodinated index p11C analog peptide (ATPYDINQM)
by unlabeled test peptides for 4 h at 20°C in the presence of
human
2m. As shown in Fig.
2, both p11C and p41A had equal, high
binding affinities for Mamu-A*01 (50% inhibitory concentration, 10 nM). Thus, the dominance of the p11C epitope and the subdominance of
the p41A epitope did not simply reflect different MHC class I binding
affinities. We therefore investigated the abilities of recombinant
modified vaccinia virus Ankara (MVA) and plasmid DNA vaccination to
elicit p11C- and p41A-specific CTL responses.
Recombinant MVA vectors expressing either SIV Gag-Pol or HIV-1 89.6 Env
under control of the same early/late vaccinia virus promoter were
constructed. Open reading frames of SIVmac239 Gag-Pol and HIV-1 89.6 Env were inserted adjacent to the modified H5 promoter in the
previously described plasmid transfer vectors pLW-9 and pLW-17
(29, 30), and recombinant MVA vectors were produced by
homologous recombination, identification by live immunostaining of
infected cell foci, and clonal isolation. Efficient expression of both
Gag-Pol and Env in cultured monkey cells was determined by
radioimmunoprecipitation, and the production of Gag particles and
surface expression and fusion competence of Env proteins were demonstrated (data not shown). Figure 3
shows the relative frequencies of p11C- and p41A-specific CTLs in four
Mamu-A*01+ rhesus monkeys vaccinated by
separate injections with these vectors. The monkeys were immunized with
108 PFU of MVA-gag pol plus 108 PFU
of MVA-env 89.6 by separate intramuscular (i.m.) injections. Four weeks after this immunization, p11C-specific CTLs were detected in
peptide-stimulated PBL of all the monkeys by both tetramer staining
(Fig. 3B) and peptide-specific functional lysis (Fig. 3C). In contrast,
these same monkeys developed significantly weaker CTL responses
specific for the p41A epitope, although one animal (H507) out of the
four did generate a readily detectable p41A-specific response. The
monkeys were boosted at week 4 with both recombinant MVA vectors, and
their PBL were assessed again for CTLs at week 10. As shown in Fig. 3D
to F, the relative epitope dominance persisted after the boosting
immunization. Overall, these monkeys vaccinated with recombinant MVA
vectors showed the same p11C epitope dominance and p41A epitope
subdominance observed in SHIV infection.
Plasmid DNA vaccines expressing either SIV Gag or HIV-1 89.6P Env in
the pV1R backbone under control of the same cytomegalovirus promoter
were then constructed (27). Equivalent expression of the
Gag and Env proteins from these plasmids was observed in transiently transfected COS cells (data not shown). Seven
Mamu-A*01+ rhesus monkeys were immunized with
5 mg of gag DNA and 5 mg of env DNA by separate
injections, and four of these monkeys also received 5 mg of an
interleukin-2-immunoglobulin plasmid as an adjuvant (4).
Four weeks after this immunization, comparable levels of p11C- and
p41A-specific CTLs were detected in peptide-stimulated PBL of all the
monkeys by both tetramer staining (Fig.
4B) and peptide-specific functional lysis
(Fig. 4C). The monkeys were boosted at weeks 4 and 8 with both plasmid
DNA vaccines, and their PBL were assessed again for CTLs at week 16. Following these boosting immunizations, codominance of both epitopes
persisted in all of the monkeys. High frequencies of both p11C- and
p41A-specific CTLs were observed by tetramer staining of freshly
obtained PBL (Fig. 4D), tetramer staining of peptide-stimulated PBL
(Fig. 4E), and peptide-specific functional lysis (Fig. 4F). These
p41A-specific CTLs were able to lyse both peptide-pulsed target cells
and virally infected target cells (data not shown). Two-tailed
Mann-Whitney tests show that the DNA vaccine-elicited p41A-specific CTL
responses, but not p11C-specific responses, were of significantly
higher frequency than those elicited by SHIV infection (P = 0.0012 for tetramer staining; P = 0.0023 for
functional lysis). Staining of these peptide-stimulated PBL using a
control Mamu-A*01/p68A tetramer consistently demonstrated a
background level of staining of <0.1% of that of CD8+ T
lymphocytes (data not shown).
These data demonstrate that the p41A-specific CTL responses that are of
low frequency in the setting of SHIV infection and recombinant MVA
vaccination are of high frequency when elicited by plasmid DNA
vaccination. However, the exact mechanism underlying these differences
remains unclear. The immunodominance of the p11C epitope does not
simply reflect inadequate Env protein expression in SHIV-infected
cells, since CTLs capable of lysing vaccinia-env-infected target cells are detectable in PBL of SHIV-infected rhesus monkeys (28). In addition, efficient expression of both Gag and
Env was observed for both the MVA and DNA constructs. The dominance of
the p11C epitope and the subdominance of the p41A epitope are also not
explained by differences in MHC-peptide affinity. Moreover, the epitope
hierarchy is not a viral isolate-specific phenomenon, since a similar
epitope dominance pattern was observed in SHIV-IIIB-, SHIV-89.6-, and
SHIV-89.6P-infected animals. It is possible that CTL epitope
immunodominance may, in part, reflect the intracellular processing or
presentation of these peptides. SHIV- and MVA-infected cells produce a
number of viral proteins that might compete or interfere with the
proteolysis, transport, or MHC class I binding of a particular epitope
(3, 32). Virally infected cells may also have nonspecific
effects on the generation of CTL epitopes. In contrast, plasmid
DNA-transfected antigen-presenting cells express only the antigenic
protein encoded by the vaccine, providing a much simpler system for the
processing and presentation of particular peptide epitopes.
It has previously been shown that CTL responses against multiple
epitopes can be elicited by a multiepitope DNA vaccine containing nine
dominant epitopes (12). Other reports have shown that DNA vaccines can elicit CTL responses specific for dominant and subdominant epitopes in a pattern similar to that observed in viral infection (6, 10, 18). Our results extend these observations by
demonstrating that vaccination with plasmid DNA can alter the natural
epitope dominance pattern and can elicit high-frequency CTL responses to an epitope that is weak in the context of infection or vaccination with a live recombinant vector.
The generalizability of this study may be limited by the fact that it
examines only two epitopes and compares inherently different expression
systems and immunization methods. However, the results suggest that DNA
vaccines may offer a potential advantage over recombinant live vector
vaccines in their ability to elicit high-frequency CTL responses to
certain epitopes that are subdominant in the context of infection. DNA
vaccines may therefore have an important benefit in diversifying and
broadening the CTL response. We have recently described the efficacy of
these DNA vaccine-elicited multiepitope CTL responses in controlling
viremia and preventing disease progression after a highly pathogenic
viral challenge (5). In the development of candidate HIV-1
vaccines, there is growing interest in strategies that utilize DNA to
prime CTL responses (11, 14, 25). The present study
provides a rationale for the use of plasmid DNA as a method of priming
a broad CTL response against multiple viral epitopes.
We acknowledge support from grants NO1-AI-65301 (M.G.L.), AI-85343
(N.L.L.), and CA-50139 (N.L.L.).
We are grateful to Nancy Miller, Carroll Crabbs, Tavis Steenbeke,
Suzanne Robinson, Meryl Forman, and Frederick Vogel for generous
advice, assistance, and reagents.
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