Augmentation of SIV DNA vaccine-induced cellular immunity by targeting the 4-1BB costimulatory molecule
Introduction
It is well-established that cell-mediated immune responses play a major role in reducing pathogenic events in viral infections. In HIV-1 or SIV infections, antigen-specific CD8+ T-cell responses are associated with the control of viral replication [1], [2], [3]. Consequently, strategies to develop a vaccine against HIV-1 aim at eliciting antigen-specific CD8+ T-cell responses. In this context, HIV-1 DNA vaccines have been shown to be effective for the induction of cellular immune responses in animal models and to elicit protection from infectious challenge in some circumstances [4]. Although several clinical studies support the safety of HIV-1 DNA vaccine strategies [5], [6], [7], [8], [9], it is clear that their potency in the clinic should be improved. A variety of combination approaches have been investigated in an attempt to enhance the cellular immune responses induced by DNA vaccines, such as the inclusion of molecular adjuvants [10] or boosting with viral vectors [11], [12], [13]. An important goal in particular is to develop improved cellular immunity with long-lasting memory T-cell responses.
T-cell activation is a complex process that requires, in addition to the recognition of specific antigen in the form of MHC/peptide expressed by antigen-presenting cells (APC), further signals from costimulatory receptors for optimal activation. Various molecules utilizing distinct mechanisms provide these signals. 4-1BB (CD137), a member of the tumor necrosis factor receptor (TNFR) superfamily, is an activation-inducible T-cell costimulatory receptor [14], [15]. It is expressed on activated CD8+ and CD4+ T cells [15], activated natural killer cells [16] and dendritic cells (DCs) [17], [18]. The natural ligand for 4-1BB, 4-1BBL, is also induced and is found on activated B-cells, macrophages and DCs [19], [20], [21]. It has been well-documented that 4-1BBL can function independently of CD28 to activate T cells [22], [23], [24]. Consequently, 4-1BB has been shown to be a potent costimulatory molecule for CD8+ T cells by enhancing cytokine production and proliferative as well as cytolytic activity both in vitro and in vivo[25], [26], [27]. A number of studies have focused on the use of agonistic anti-4-1BB monoclonal antibodies (mAb) in order to stimulate 4-1BB and to enhance CD8+ T-cell responses in various mouse model systems. Those studies have shown that anti-4-1BB mAb can have an impact on CD8+ T-cell-mediated anti-viral and anti-tumor immunity [23], [28], [29], [30], augmenting T-cell immunity, survival or tumor regression [28], [31], [32], [33]. To date, limited research has been carried out to investigate the effect of anti-4-1BB mAb in non-human primates. Hong et al. [34] reported that a humanized anti-4-1BB mAb suppresses IgG production against a T-cell-dependent antigen. To our knowledge, there has been no study evaluating the effect of anti-4-1BB mAb on antigen-specific cellular immunity in the macaque model.
In the present study, we sought to determine whether anti-4-1BB mAb could be utilized to enhance cellular immunity induced by a DNA vaccine. We examined the in vivo effect of anti-4-1BB mAb on the antigen-specific cellular immune response in macaques immunized with a DNA vaccine encoding the SIV Gag antigen (pSIVgag). We hypothesized that naïve T cells can be primed against the antigen encoded by the DNA vaccine and then costimulated with anti-4-1BB antibodies, which in turn may enhance antigen-specific cellular immune responses. We demonstrate that anti-4-1BB mAb can be powerful adjuvants for antigen-specific cellular immunity in macaques.
Section snippets
In vitro costimulation using agonist anti-4-1BB mAb enhances IFN-γ production by macaque and human PBMC
4-1BB is an inducible costimulatory molecule [14], [15], meaning expression is increased following T-cell stimulation. In order to verify a similar expression pattern of 4-1BB in cynomolgus macaques, PBMC obtained from healthy animals were incubated for 5 days with or without ConA (5 μg/mL). Following incubation, cells were washed and stained for expression of T-cell markers as well as 4-1BB expression. Our results demonstrate a significant increase in 4-1BB expression on T cells following
Discussion
To date, there has been no study evaluating the effects of anti-4-1BB mAb on the antigen-specific cellular response in DNA-immunized macaques. We report that the administration of anti-4-1BB mAb after the first pSIVgag immunization was able to augment the DNA vaccine-induced antigen-specific cellular immunity. Other groups have reported that anti-4-1BB mAb in vivo have a favored role as costimulation molecules for CD8+ T cells, promoting the proliferation of CD8+ T cells, amplification of
Animals
Twenty-four cynomolgus macaques (Macaca fascicularis), eight females and 16 males from 2.8 to 5.6 kg, were included in the study. Animals were purchased from Charles River BRF (Houston, TX) and housed at the Bristol–Myers Squibb (BMS) Lawrenceville Facility (Princeton, NJ). The BMS Animal Care and Use Committee addresses Animal Facilities, Contract Facilities, and Animal Suppliers. In addition, the animal facility is accredited by the Association for Assessment and Accreditation of Laboratory
Acknowledgements
We are grateful to Emel Girit, Robert Scalese and Susan Barbour for excellent technical assistance. This work was supported by grants from NIH to D.B.W. including an HVDDT.
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