The main finding in this study is that immunization of macaques with a live attenuated vaccine, NYVAC/SIVgpe, by either a mucosal (intrarectal and intranasal) or systemic (intramuscular) route results in the appearance of Gag181-189 CM9 tetramer-positive CD8+ cells in mucosal tissues. Furthermore, intrarectal challenge of such immunized macaques with SIVmac251 followed by sacrifice of the animals at 48 h after challenge led to the expansion of tetramer-positive CD8+ cells in mucosal tissues in all macaques regardless of the route of immunization. This confirmed that a systemic (intramuscular) immunization led to mucosal CD8+ T-cell responses, as did mucosal immunization.
These results are somewhat at odds with studies reviewed above (4
) that show that the route of induction determines whether or not there is a mucosal immune response and that, in fact, mucosal immunization is necessary for a mucosal response. Thus, intramuscular and intradermal immunization with naked DNA generates systemic immune responses but is unable to confer protection at mucosal sites (36
). Similarly, subcutaneous immunization in mice with HIV-1 gp160 expressing recombinant vaccinia virus induced cytotoxic T lymphocytes only in the spleen but not in Peyer's patches or intestinal lamina propria (3
However, it should be noted that systemic immunization with recombinant vaccinia virus vac-gp160 protected three of four macaques against intrarectal challenge with SIVmne
virus 47 (39
) or, in another model, cats against feline immunodeficiency virus (38
). Similarly, such protection has been achieved with live attenuated virus (41
) and whole killed virus (10
). Finally, in the case of the NYVAC/SIVgpe
vaccine, intramuscular immunization was previously shown to be able to protect 50% of the macaques that were challenged intrarectally with SIVmac251
from high viremia ((7
) and an even higher number of macaques when a DNA prime/NYVAC/SIVgpe
boost was used (18a
It should be noted that in the studies where systemic immunization led to at least some degree of mucosal immunity, live virus vaccines were used. Such vaccines, as opposed to peptide vaccines, may be more capable of leading to the appearance of immune cells at mucosal sites, either because of antigen circulation or because of the traffic of cells that take up antigen.
Several additional conclusions concerning the responses elicited by immunization by different routes could be drawn from this study. First, tetramer-positive CD8+
T cells were found in the blood in all macaques regardless of the route of immunization. This is in agreement with data obtained in animals infected by different routes (18
). Second, intrarectal immunization induced CD8+
T cells mainly in mucosal tissue. Thus, while intramuscular and intranasal immunization led to some level of tetramer-positive CD8+
cells in systemic tissues (spleen, iliac lymph node), intrarectal immunization led to little or no tetramer-positive CD8+
cells at these locations. This finding is in agreement with previous studies showing that intrarectal immunization is less effective in inducing systemic responses (31
). Since intrarectal immunization with a multicomponent-peptide HIV vaccine led to cytotoxic T lymphocyte responses in the spleen only when cholera toxin was used as an adjuvant (23
), it may be that systemic responses following mucosal immunization require the use of adjuvants.
Intranasal immunization has been proven by numerous reports to be particularly effective in inducing immune responses in the female genital tract (14
). Studies done in macaques with attenuated vaccinia virus that express Env and Gag proteins of SIV or recombinant live attenuated poliovirus expressing the SIV proteins p17gag
confirmed these findings regarding antibody responses (12
). Studies in macaques that looked for cellular responses in the cervicovaginal tract following intranasal immunization, to our knowledge, have not been performed. In this study, intranasal immunization induced T-cell responses in both mucosal and systemic compartments. In addition, cytokine production (IFN-γ or TNF-α) in response to stimulation with the specific peptide Gag181-189 CM9 was high in both animals immunized intranasally.
One caveat of the results of this study is that, while responses to immunization and challenge were measured, protection against infection was not. Thus, it may be that mucosal immunization is still necessary, even with live virus vaccines, to achieve optimum protection. In a previous study with ALVAC/SIV, another poxvirus-based vaccine, mucosal/systemic immunization was not shown to provide better protection than systemic immunization alone, but in this study, the mucosal and systemic routes of immunization alone were not directly compared (37
). Clearly, studies in larger numbers of macaques investigating protection achieved with different routes of immunization are now warranted.