rs3764880 — TLR8 A1G

TLR8 A1G — An X-Linked Immune Tuner with Opposite Effects in Men and Women

Toll-like receptor 8 (TLR8) is an endosomal sentinel of the innate immune system that detects single-stranded RNA from invading pathogens — including RNA viruses and the RNA-rich debris shed by mycobacteria such as Mycobacterium tuberculosis. TLR8¹¹ TLR8
Toll-like receptor 8, located on chromosome X at Xp22.2, expressed predominantly in monocytes, dendritic cells, and lung tissue sits inside the membranes of intracellular vesicles where it scans the contents of phagocytosed material for pathogen signatures. When activated, it triggers NF-κB and interferon signaling cascades that orchestrate the early inflammatory response. The A1G variant (rs3764880) sits at the very beginning of the TLR8 gene — at the initiator codon — and alters how the cell translates two different versions of the TLR8 protein. Because TLR8 is on the X chromosome, men carry only one copy of this gene, while women carry two. This dosage asymmetry is the key to understanding why the same variant can be protective in one sex and a risk factor in the other.

The Mechanism

The TLR8 gene produces two protein isoforms — TLR8v1 (the shorter form, associated with positive regulation of TLR8 signaling in differentiated monocytes) and TLR8v2 (the predominant form in most immune cells). The A1G variant substitutes a guanine (G) for the adenine (A) at the very first position of the coding sequence, changing the methionine start codon to valine (p.Met1Val) in TLR8v2. A 2010 functional study²² A 2010 functional study
Gantier et al. Immunology and Cell Biology, PMID 20652908 showed that this does not abolish TLR8 protein production but instead shifts the relative translation ratio between the two isoforms, fine-tuning how much of each variant is made. The net effect is a subtle but reproducible modulation of downstream NF-κB activation and cytokine secretion profiles in monocytes and neutrophils — not an on/off switch, but a dial adjustment.

Because TLR8 is X-linked, the allele dosage differs between sexes in a fundamental way. Males have a single allele (either A or G), while females have two alleles and can be homozygous (AA or GG) or heterozygous (AG). The heterozygous state in females means they express both TLR8 isoform ratios simultaneously, potentially creating a more balanced signaling profile. Males, with only one copy, experience the full directional effect of whichever allele they carry. This X-linked dosage asymmetry appears to explain the consistent pattern of sex-specific findings seen across independent tuberculosis and HIV studies.

The Evidence

The tuberculosis associations are the best-replicated findings for this variant. A 2015 South African case-control study³³ A 2015 South African case-control study
Salie et al., 729 TB cases and 487 controls found that rs3764880 showed opposite effects by sex: females with the G allele were more susceptible to TB (OR=1.42, p=0.011), while males with the G allele were protected (OR=0.75, p=0.036). This sex-opposing pattern was replicated in a Moldavian cohort of 272 TB cases and 251 controls⁴⁴ a Moldavian cohort of 272 TB cases and 251 controls
Varzari et al. 2019, PMID 30529560, where males carrying the minor allele had notably lower TB risk (OR=0.44, p=0.0087). In a large Chinese Han cohort (584 cases and 608 controls, validated in a second independent sample)⁵⁵ a large Chinese Han cohort (584 cases and 608 controls, validated in a second independent sample)
Wang et al. 2018, PMID 30424735, the A allele was protective in males (OR=0.58, 95% CI 0.37–0.91), while the homozygous AA genotype actually increased pulmonary TB risk in females (OR=4.81) — a striking reversal. A meta-analysis of 29 studies covering 17,804 individuals confirmed that TLR8 rs3764880 A alleles were associated with overall tuberculosis susceptibility⁶⁶ TLR8 rs3764880 A alleles were associated with overall tuberculosis susceptibility
Sun et al. 2015, PMID 25877346, with the male-specific protective effect appearing in subgroup analyses.

For HIV, the G allele appears protective against disease progression. An 18-month cohort study of 782 HIV-positive adults and 550 controls⁷⁷ An 18-month cohort study of 782 HIV-positive adults and 550 controls
Oh et al. 2008, PMID 18605904 found that the A1G polymorphism was significantly protective against HIV disease progression, with laboratory evidence showing impaired NF-κB activation in G-allele carriers — a finding consistent with the reduced cytokine storm hypothesis. However, the X-linked complexity reasserts itself in early life: a perinatal HIV cohort of 368 Kenyan infants⁸⁸ a perinatal HIV cohort of 368 Kenyan infants
Beima-Sofie et al. 2013, PMID 24037211 found that female infants carrying the G allele had a 0.78 log₁₀ copies/ml higher peak viral load (p=0.0009), suggesting that at least in the context of neonatal HIV exposure, G allele expression in females may be disadvantageous.

The current evidence base is strong — multiple independent replication cohorts across diverse populations have confirmed the sex-specific tuberculosis associations, and the functional mechanism linking the variant to isoform ratio changes has been demonstrated in primary human cells. The overall picture, however, is complex enough that no simple "good allele / bad allele" framing applies.

Practical Implications

For men, the A allele (the reference, more common in most populations) is associated with lower TB risk in multiple independent studies, while the G allele may carry modest increased susceptibility. In male genotyping results (hemizygous — one copy only), the A call represents the protective state. For women, the relationship is inverted and dose-dependent: heterozygous AG women appear to have an intermediate or near-neutral risk profile, while homozygous AA women show paradoxically elevated TB risk in at least one large study. Homozygous GG women appear to carry the highest TB susceptibility in studies where female risk is elevated.

TB risk from this SNP is one factor among many. BCG vaccination status, TB exposure history, nutritional status, and overall immune competence dominate risk. This variant most relevantly informs awareness of susceptibility in endemic settings and the priority of maintaining BCG coverage.

Interactions

TLR8 signals through the same MyD88-dependent pathway as TIRAP and TLR4. The TIRAP rs8177374 (Ser180Leu)⁹⁹ rs8177374 (Ser180Leu) variant modulates how efficiently TIRAP bridges activated TLR receptors to MyD88 — the immediate downstream partner that TLR8 also recruits after pathogen RNA recognition. While TLR8 uses the MyD88-dependent pathway rather than the TIRAP-bridged TLR2/TLR4 cascade, both converge on NF-κB and cytokine production. Individuals carrying variants at both TIRAP and TLR8 may experience compounded modulation of mycobacterial immune responses.

TLR7 (rs179008), located adjacent to TLR8 on the X chromosome, is a closely related receptor that also recognizes single-stranded RNA and is in strong functional interaction with TLR8 for antiviral immunity. The two receptors share endosomal localization and compete for the same ligands.