CpG Motifs and TLR9 Activation: A Comparative Analysis of Wild AAV, Recombinant Vectors, and other DNA Viruses

Abstract

Adeno-associated virus (AAV) vectors are widely used to deliver genes to the central nervous system, but they still come with important limitations. One major issue is that they can activate the immune system and, in some cases, contribute to neuronal toxicity. Recent studies have shown that this response can be triggered when Toll-like receptor 9 (TLR9) detects unmethylated CpG motifs within the AAV genome. In this study, I analyzed CpG frequency and distribution across wild AAV serotypes, recombinant AAV components, and other DNA viruses. CpG enrichment was first assessed based on observed versus expected CpG frequency, and I further used the RF3 metric to evaluate CpG content in the context of TLR9 immunostimulatory potential. I compared wild AAV genomes to randomized control sequences that preserve nucleotide composition but remove biological structure. In addition, wild AAV genomes showed reduced RF3 values, suggesting a depletion of CpG motifs associated with TLR9 activation. This pattern was consistent across all serotypes and was statistically significant. When I analyzed different regions of the AAV genome, I found that CpG distribution is not uniform. Coding regions (rep and cap) showed consistent and constrained RF3 values, while inverted terminal repeats (ITRs) showed much higher variability. Then, I compared these patterns to recombinant AAV components and a panel of DNA viruses. I also included DNA viruses as a comparison group to see whether the patterns seen in AAV were part of a broader viral trend. This was important because different DNA viruses have different life cycles, genome structures, and relationships with the host. Recombinant AAV elements showed a wide range of RF3 values depending on the type of component. In particular, transgenes often had higher CpG content, suggesting that they may contribute more strongly to predicted TLR9 activation in engineered vectors. In contrast, oncogenic DNA viruses had significantly lower RF3 values than nononcogenic viruses. This suggests that CpG depletion may be related to viral persistence and immune evasion, especially in viruses that remain in the host long term. Importantly, this pattern did not seem to be explained by genome length or GC content alone. Although CpG frequency can be affected by overall nucleotide composition, viruses with similar GC content still showed different RF3 values. Oncogenic viruses also continued to show lower CpG enrichment even when compared within similar GC ranges. Overall, these results suggest that wild AAV genomes may have been shaped by evolutionary pressure to reduce CpG motifs and avoid TLR9 mediated immune detection. In contrast, recombinant AAV vectors are built from different engineered parts, some of which may reintroduce CpG high sequences. These findings show why CpG content should be considered in AAV vector design and may help support the development of safer and more effective gene therapy strategies.