Potent antibodies against SARS-CoV-2 can be produced without T follicular helper cells, suggests study

T follicular helper (Tfh) cells are indispensable to the generation of long-lasting immune memory. These are involved in the production and maintenance of germinal centers, which ensure antibody maturation and the creation of effective memory B cells.

There is a correlation in coronavirus disease 2019 (COVID-19) patients between neutralizing antibody titers and the concentration of circulating Tfh cells. However, it is less certain that Tfh concentration can be correlated with disease severity. Some severe COVID-19 cases with deficient Tfh cells due to virus-induced inflammation have been noted, though functions such as antibody class switching and B cell –T cell interactions were found to still occur in the secondary lymphoid organs.

Study: High-affinity, neutralizing antibodies to SARS-CoV-2 can be made in the absence of T follicular helper cells. Image Credit: Design_Cells / Shutterstock

It has been uncertain whether antibodies produced via this non-canonical pathway are protective against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In a new study, recently released as a preprint on the bioRxiv*server, the neutralization potency of these antibodies is compared with those traditionally produced in germinal center-based antiviral responses.

How was the study performed?

Previous studies have demonstrated that CD4⁺ T-cells are necessary to produce neutralizing antibody titers against SARS-CoV-2, though Tfh CD4⁺ T-cells are needed to induce high-affinity class-switched antibodies. Non-Tfh cells can promote effective antibody production in some contexts. However, many studies have shown that long-lasting immunity is Tfh dependent, including following the administration of a vaccine where Tfh cell populations indicate vaccine efficacy. To test whether non-Tfh CD4⁺ T-cells could compensate for Tfh cell loss in viral infection, the group applied SARS-CoV-2 and influenza A to Tfh deficient mice, characterizing the type, affinity, function, and titer of the produced antibodies.

Several types of mice with differently deactivated T-cell production were utilized: Bcl6^fl/fl mice, with impaired Tfh cell differentiation; Bcl6^fl/flCd4^Cre mice, which specifically and completely lack Tfh cells; and Ciita^−/− mice, which completely lack all CD4+ T-cells. Human ACE2 was made to overexpress in the respiratory tract of the mice two weeks before application of the virus, and antibodies were collected a further two weeks following infection. It was found that the mice produced high levels of Tfh independent class-switched high-affinity antibodies even in the absence of Tfh cells, most likely produced in the lymph nodes. The formation of germinal center B-cells was severely impaired in the Bcl6^fl/fl and Bcl6fl^/flCd4^Cre mice, though the former still produced high levels of spike protein-specific IgG antibodies. Both mice produced notably more IgG antibodies than the Ciita^−/− mice, while spike protein-specific IgM was unaffected by the removal of Tfh cells, both in SARS-CoV-2 and influenza A infection.

Alternative routes to IgG production

The total number of activated CD4⁺ T-cells was consistent between Bcl6^fl/fl and Bcl6^fl/flCd4^Cre mice, though more of them were T-helper (Th1) cells in the latter, as expected due to the loss of Tfh cell production. Effector molecules ascribed to Tfh cells, CD40L and interleukin-21, were also found to be produced by Th1 cells in a shared manner between the T-cell types in Bcl6^fl/fl mice and as the major producer in Bcl6^fl/flCd4^Cre mice, which completely lack Tfh cells.

The group next analyzed whether Th1 cells are correctly positioned to support B-cells, finding that they are colocalized with B-cells in both Bcl6^fl/fl and Bcl6^fl/flCd4^Cre mice. This suggests that Th1 cells promote antibody production both in the presence or absence of Tfh cells. The antibodies produced by these mice were high-affinity towards the spike protein and receptor-binding domain (RBD) of SARS-CoV-2, reaching a peak 14 days post-infection and persisting to at least 49 days, while IgG antibodies produced in Ciita^−/− mice were much lower affinity and did not persist for nearly as long.

The Tfh-independently (Bcl6^fl/flCd4^Cre mice) produced antibodies lacked the IgG1/IgG3 subclasses. In tests, the neutralization potency of Bcl6^fl/fl mice sera was seen to be higher, mainly due to the higher titers of spike protein-specific IgG. However, the neutralization potency of each individual antibody was found to be greater in Bcl6^fl/flCd4^Cre mice sera when normalizing the titer levels. Finally, the group tested the neutralization capacity of the collected antibodies against the SARS-CoV-2 variant of concern B.1.351, finding that efficacy was maintained both in antibodies produced via the Tfh cell route or independently of it.

IgG1 and IgG2 bind with Fc receptors, key immune regulatory modulators, with the latter being essential to viral clearance in vitro. As Th1 cells could produce IgG2, the group expects sufficient levels of antibodies with Fc effector function to be produced, even in the absence of Tfh cells. Unlike SARS-CoV-2, influenza failed to produce high-affinity Tfh independent antibodies. Profiling the antibodies generated against SARS-CoV-2 by Th1 revealed that most epitopes were towards highly conserved regions across human coronaviruses, which the authors suggest could explain the possible lesser need for antibody affinity maturation in those generated by Th1 in the lymph nodes.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Michael Greenwood

Michael graduated from Manchester Metropolitan University with a B.Sc. in Chemistry in 2014, where he majored in organic, inorganic, physical and analytical chemistry. He is currently completing a Ph.D. on the design and production of gold nanoparticles able to act as multimodal anticancer agents, being both drug delivery platforms and radiation dose enhancers.