Two main mechanisms are involved in presenting antigens to induce CD8⁺ T cell responses, namely direct and cross priming. Systemic treatment of mice with a toll-like receptor 9 agonist, an oligodeoxynucleotide containing a CpG motif (CpG-ODN), has been shown to inhibit cross priming, but not direct priming. Therefore this method has been applied to examine priming mechanisms in various disease models. Here, we set out to use this tool to study priming of CD8⁺ T cells during vaccinia virus (VACV) immunisation in C57BL/6 mice. Firstly, we confirmed that CpG-ODN eliminated cross presentation of antigens from VACV-infected cells. However unexpectedly, CpG-ODN suppressed CD8⁺ T cell responsesagainst most VACV antigens induced during local dermal VACV immunisation. Although CpG-ODN lowered the virus load in the draining lymph nodes, this effect did not account for the reduction of anti-VACV CD8⁺ T cell immunity. Instead, we discovered that CpG-ODN inhibited CD8⁺ T cell response against a recombinant antigen that requires direct presentation when expressed from VACV. Moving to a systemic intraperitoneal immunisation model, we found that inhibition of priming by CpG-ODN could be overcome if a robustly-replicating strain of VACV was used. Likewise, simply increasing the dose of antigen allowed cross priming to be detected in CpG-treated mice. In summary, CpG-ODN does not dissect priming pathways, but rather reflects antigen dose. More importantly, this study suggests that the professional antigen-presenting cells involved in cross priming also play an essential role in direct priming.