Defective T_FH cell checkpoint in SLE

T follicular helper (T_FH) cells are an integral part of barrier immunity and immune responses to infection, but can also aid autoantibody production in autoimmune diseases such as systemic lupus erythematosus (SLE). The results of a new study suggest that the dysregulation of T_FH cells that occurs in SLE could be the result of a defective T_FH cell checkpoint.

Credit: Paul Italiano/Alamy Stock Photo

In the gut, T_FH cells respond to extracellular ATP released by microbes via P2X purinoceptor 7 (P2X7) signalling, which triggers cell death pathways in the presence of high concentrations of extracellular ATP. In the absence of P2X7, T_FH cells do not undergo cell death and provide deregulated help to B cells, resulting in increased production of antibodies. However, it is unclear if T_FH cells in germinal centres react to extracellular ATP in patients with SLE, and whether P2X7 signalling affects autoantibody production.

“We tested whether P2rx7^−/− mice showed some defect in P2X7-mediated control of T_FH cells in the ATP-rich inflammatory environment that is generated in pristane-induced lupus (PIL),” explains co-corresponding author Fabio Grassi. “This experimental model allowed us to address T_FH cell-dependent pathogenetic aspects in mice on the C57BL/6 background, including P2rx7^−/−, Icos^−/−P2rx7^−/− and Cd4-Cre P2rx7^fl/fl mice, which would have been extremely difficult to address in murine models of spontaneous lupus.”

P2rx7^−/− mice with PIL had more severe disease, an increased number of T_FH cells in germinal centres and greater autoantibody production than wild-type mice with PIL, suggesting a role for P2X7 in regulating T_FH cell functions during disease. Icos^−/−P2rx7^−/− mice, which lack T_FH cells and are unable to form germinal centres, produced fewer autoantibodies than P2rx7^−/− mice upon disease induction. Further experiments in germinal centre-competent mice that lack P2X7 in T cells confirmed autoantibody production in P2rx7^−/− mice with PIL to be T_FH cell-dependent.

The number of circulating T_FH cells able to respond to stimulation with a P2X7 agonist ex vivo was reduced in patients with SLE compared with healthy individuals and inversely correlated with the frequency of circulating T_FH cells. The amount of P2RX7 mRNA in circulating T_FH cells from patients with SLE was also reduced compared with healthy individuals, suggesting that downregulation of P2RX7 might be linked to the increased number of T_FH cells in patients with SLE.

Interestingly, T_FH cells from patients with primary antiphospholipid syndrome (APS) did not share the dysfunctional phenotype of T_FH cells from patients with SLE. “Patients with primary APS and SLE share SLE-susceptibility genes, yet patients with primary APS do not develop complete SLE,” states co-corresponding author Pier Luigi Meroni. “Our study unravels a pathogenic pathway that operates in patients with SLE, but not in patients with primary APS.”

“This study provides new mechanistic insights for P2X7-mediated regulation of T_FH cell homeostasis and function,” comments Di Yu from the John Curtin School of Medical Research in Canberra, Australia, who was not involved in this project. “In my opinion, it strongly suggests that the defect in P2X7-mediated inhibition of T_FH cell generation can promote lupus, as shown in both a pristane-induced mouse model and using human samples. However, the reduced expression of P2X7 is likely to be one of many reasons for the hyperactive phenotype of T_FH cells in SLE.”

Importantly, P2X7 signalling seems to affect the expansion of T_FH cells in autoimmunity, but not during immunization responses. No difference was seen in the number of antigen-specific CD4⁺ T cells after immunization with ovalbumin in mice adoptively transferred with ovalbumin-peptide-specific T_FH cells from either congenic P2rx7-deficient or wild-type mice.

downregulation of P2RX7 might be linked to the increased number of T_FH cells in patients with SLE

“The identification of P2X7 as a selective checkpoint inhibitor of T_FH cells suggests that restoring P2X7 activity in SLE could limit the progressive amplification of pathogenic autoantibodies but have no suppressive effects on adaptive immune responses against pathogens,” says Meroni. “Rather, improving P2X7 activity should beneficially influence the reduced responsiveness of patients with SLE to vaccines.”