The results from this study clearly show that length of exposure to Depo had a significant influence on immune responses following immunization with an attenuated strain of HSV-2. The changes in immune responses resulted in mice being protected or not from subsequent challenge with wild-type virus. Previous studies have shown that immunization with TK−
HSV-2 3 to 5 days following Depo treatment leads to complete protection against subsequent challenge (16
). In our study we found similar results, where shorter exposure to Depo (5 days) induced immune responses adequate to protect against subsequent i.vag. challenge. In contrast, longer exposure (2 weeks) to Depo inhibited induction of immune responses and failed to provide protection against subsequent genital challenge with wild-type HSV-2. Following immunization, there was a lack of induction of IFN-γ in vaginal secretions of Depo 15 mice, indicating that local innate responses were compromised. There was also a lowered Th1 response in the local draining LN of Depo 15 mice. Following challenge with wild-type HSV-2, inadequate protection in the Depo 15 group correlated with the absence of local antibody responses in the genital tract compared to the Depo 5 group.
While previous clinical and experimental studies have clearly shown that progesterone increases susceptibility to STDs, the present study starts to delineate the effects on immune responses. The most intriguing observation was the suppressive effect of progesterone on innate immune responses following immunization with TK−
HSV-2. Previous studies have shown that in response to primary vaginal infection with HSV-2 there is a biphasic secretion of IFN-γ which plays an important role in clearance of virus (18
). The early peak of IFN-γ seen in vaginal secretions is part of the innate response produced primarily by NK and NKT cells in response to the viral infection (18
). Studies in other infection models have shown that IFN-γ produced by NK cells plays an important role in skewing the developing immune response to Th1 type (23
). Local IFN-γ may also play an important role in activating the antigen-presenting cells and upregulating major histocompatibility complex class II on antigen-presenting cells, to enhance presentation of viral antigens (4
). This early IFN-γ response is absent in animals with prolonged exposure to Depo, indicating a possible deficiency in the initiation of an innate response to HSV-2 immunization. In other studies from our group, mice that genetically lack the ability to mount this initial innate response (IFN-γ−/−
mice) also showed a significant increase in susceptibility to HSV-2 genital infection (2
The progesterone effect was also evident on adaptive immune responses to HSV-2 immunization. The Th1/Th2 ratios were significantly lowered in Depo 15 mice following immunization, as were the local antiviral IgA and IgG antibody levels following challenge. One possibility is that the lack of the initial innate response affected subsequent induction of protective adaptive immune responses. The absence of IFN-γ in the early innate response could alter the Th1 response to more of a Th2 response and consequently affect mucosal antibody responses. On the other hand, both the innate and adaptive responses may be independently inhibited by Depo. It was recently reported that, when Depo was administered in previously immunized mice, the IgG and IgA antibody levels were lowered dramatically in the vaginal secretions, indicating that Depo may directly inhibit local antibody levels (10
The cellular mechanism by which longer exposure to progesterone downregulates the protective immune response is not clear. Our data suggest that the effect requires more than 5 days of exposure to Depo. Results from other studies indicate that progesterone can exert a suppressive effect on neutrophils (19
) and NK cell function (5
). It is therefore possible that prolonged exposure to Depo may affect the ability of these cells to secrete appropriate chemokines and cytokines to initiate an antiviral innate immune response. Clinical studies in tumor research have also shown that continuous or intermittent scheduled administration of medroxyprogesterone acetate can lead to cell cycle arrest of human hematopoietic progenitors (22
). If this mechanism is active in this HSV-2 model, then under the influence of long-term exposure to Depo progenitor immune cells may not be able to proliferate, resulting in a reduced antiviral response. We are currently investigating this possibility.
The finding that progesterone enhances susceptibility to sexually transmitted viruses has been demonstrated by a number of other studies (12
). In the rhesus macaque model of SIV it has been shown that progesterone implants enhance SIV vaginal transmission and virus load (15
). The increase in transmission in this and other studies has been attributed to the atrophy of vaginal epithelium under the influence of progesterone. The thin epithelium may allow the virus to easily penetrate the epithelial layer and/or establish infection in susceptible target cells under the epithelium. While the present study was done in mice, the results from this study may have important implications for women using Depo as a method of contraception. In addition to thinning the genital epithelium, if continuous use of Depo also affects immune responses in the genital tract of women, then it may further increase the risk of sexually transmitted infections, including HIV. This is especially important in adolescent girls who represent the group at highest risk of contracting STDs (3
). Studies need to be done to further examine if women who are on Depo or other hormonal therapies have altered susceptibility to STDs and/or immune responses to vaccines. If so, additional precautions such as barrier methods of contraception are important for women who are currently using hormonal contraceptives and therapies, to protect themselves against increased risk of transmission.
The ultimate goal for a successful STD vaccine would be to induce sterile immunity in the genital tract. Previous studies have shown clearly that, in both animal models and women, reproductive tract immune responses are regulated by the sex hormones estradiol and progesterone (29
). It was previously shown that estradiol and progesterone regulate susceptibility and immune responses in a rat model of C. trachomatis
) and a mouse model of HSV-2 (10
). The present study extends these findings by showing that hormonal treatments such as Depo may also affect the ability of the immune system to fight sexually transmitted viral infections. These studies emphasize the need to consider the hormonal status of women in order to develop effective STD vaccine strategies. It also has important implications for women who are currently on hormonal therapies.