Opening Paragraph (serves as abstract for submission) and Body 1 2The Intact Proviral DNA Assay (IPDA) was developed to address the critical need for a 3 precise and scalable method for intact HIV reservoir quantification 1 . This duplexed droplet 4 digital PCR (ddPCR) assay simultaneously targets the HIV Packaging Signal (Ψ) and the Rev 5Responsive Element (RRE) within Envelope (env) to distinguish genomically intact proviruses 6 against a large background of defective ones 2 . The IPDA requires less time, resources, and 7 biological material than the gold standard for replication-competent HIV reservoir measurement, 8the Quantitative Viral Outgrowth Assay (QVOA) 3 , and is being adopted in research and clinical 9 studies 4-7 . In our cohort of HIV-1 subtype B-infected individuals from North America however, 10 the IPDA yielded a 28% failure rate due to HIV polymorphism. We further demonstrate that 11 within-host HIV diversity can lead the IPDA to underestimate intact HIV reservoir size, which 12 could negatively impact clinical trial results interpretation. While the IPDA represents an 13 important methodological advance, HIV diversity should be addressed before its widespread 14
adoption. 15We applied the IPDA to 46 HIV subtype B-infected, virally-suppressed individuals from 16North America, yielding a median of 29 (interquartile range [IQR] 0-93) intact 17 proviruses/million CD4+ T-cells (Extended Data Figure 1). Of note, the IPDA did not detect any 18 intact (i.e. Ψ and env double-positive) proviruses in 17 participants (37%). In four of these 19 individuals, both Ψ-and env-single-positive proviruses were detected, suggesting a true negative 20 result (Extended Data Figure 3) In the remaining 13 individuals however, the IPDA did not 21 detect Ψ-and/or env-single-positive proviruses above background levels despite recovery of 22 replication competent HIV in 11/11 cases where QVOA was performed, suggesting that the 23 assay failed to detect autologous proviruses. Specifically, in eight of these participants only Ψ-24 3 positive proviruses were detected, in four only env-positive proviruses were detected, and in one 1 participant no proviruses were detected ( Figure 1B; Extended Data Figure 3). This non-detection 2 rate is consistent with Gaebler et al who reported a lack of detectable Ψ and env double-positive 3 proviruses in 2/6 individuals (33%) using a quadruplexed qPCR assay that employs the IPDA 4 probes 8 . Near-full-length single-genome proviral sequencing performed on IPDA env-negative 5 participant BC-004 revealed mismatches to the env probe, where in silico-predicted reservoir 6 distributions that took these polymorphisms into account ( Figure 1C, left) differed markedly 7 from the experimentally-obtained result (center). Substituting an autologous env probe rescued 8 detection to in silico-predicted levels ( Figure 1C, right), confirming that HIV polymorphism can 9 cause the IPDA to fail. HIV sequencing revealed mismatches in the probe and/or at the 3' end of 10 a primer in all cases of presumed ass...