Helper T Cell Response to Factor VIII In Vivo Requires Several Anatomically Distinct Types of Antigen Presenting Cells
Authors: Kaczmarek Radoslaw , Pineros Annie R , Arvin Matthew Carl , Bertolini Thais , Camire Rodney M. , Herzog Roland W 
Published: November 2021

Abstract
Inhibitor formation is the most serious complication of factor (F)VIII replacement therapy for hemophilia A. It has long been clear that FVIII inhibitors arise in a CD4 + T cell-dependent manner, but early events in the immune response leading to MHC-II presentation and CD4 + T cell activation remain obscure. Several types of antigen presenting cells (APCs) have been implicated in the uptake of FVIII, but their relevance in T cell activation has been unclear. This study aimed to pinpoint the roles of APCs in priming FVIII-specific CD4 + T cells in vivo.
Several transgenic strains of mice on the C57BL/6J background were employed to perform in vivo antigen presentation assays involving one intravenous (IV) injection of 5 µg FVIII-OVA fusion protein (FOVA), which contained an MHC-II I-A b peptide epitope of chicken ovalbumin in place of the B domain of FVIII, and an adoptive transfer (via IV injection on the following day) of 5E6 transgenic OT-II CD4 + T cells, which express chicken ovalbumin-specific T cell receptor (TCR). Before adoptive transfers, OT-II T cells were stained with CellTrace Violet (CTV). Spleens were collected for flow cytometry analyses four days after the adoptive transfer. FOVA shows identical specific activity to that of B domain deleted FVIII.
CD11c-DTR/GFP mice (which express diphtheria toxin receptor and green fluorescent protein under the CD11c promoter) (n=3) received two intraperitoneal (IP) injections of 100 ng diphtheria toxin (DT) or PBS only (n=3) one day before and on the day of FVIII-OVA injection to deplete CD11c high cells (i.e. dendritic cells, DCs, marginal zone, sinusoidal and metallophilic macrophages, MFs). DT treatment completely abrogated T cell proliferation in the animals (p=0.03), while on average 11.1% of CD4 +CTV + cells from the control animals were progenies of the transferred cells.
XCR1-DTRvenus mice (which express DT receptor and the Venus variant of yellow fluorescent protein under the XCR1 promoter) were similarly treated with DT or PBS (n=4/group) one day before FOVA injection and one day after adoptive transfer to deplete type I conventional DCs (which make up ~80% of XCR1 + cells). XCR1 + cell depletion reduced T cell proliferation by ~50% (p=0.02). CD4 +CTV + cells from the DT-treated and control mice included 19.4% and 38.6% progenies, respectively.
Hemophilia A (F8e16-/-) mice received 100 µg of marginal zone (MZ) B cell-depleting antibodies anti-CD11a and anti-CD49d (n=4) or isotype control antibodies (n=3) four and two days before FOVA injection. MZ B cell depletion completely abrogated T cell proliferation (p=0.02), while on average 31% of CD4 +CTV + cells from the control animals were progenies of the transferred cells. In another experiment, hemophilia A mice were injected with GdCl 3 (n=4), which inactivates MFs, or PBS (n=3) one day before and on the day of FOVA administration. MF inactivation completely abrogated T cell proliferation in all but 1 animal (p=0.03), while on average 15.9% of CD4 +CTV + cells from the control animals were progenies of the transferred cells.
To visualize APC-T cell interactions in vivo, we performed multiphoton (MP) intravital microscopy (IVM) of inguinal lymph nodes (LNs) in CD11c-DTR/GFP mice. The animals received adoptive transfers of 1E7 CTV-stained OT-II CD4 + T cells IV ~24 hours and 5 µg FOVA 20, 5 or 1 hour before IVM delivered intradermally (ID) to target the skin-draining LNs. Control animals received adoptive cell transfer only. For IVM, the LNs were surgically exposed in live, anesthetized animals. Five hours after FOVA injection, multiple CTV + OT-II T cells formed clusters around GFP + cells throughout the T cell zone with several motile T cells surveying the B cell follicle. Twenty hours after FOVA injection, CTV + T cells densely populated and demarcated the T-B border in the LNs. CTV + T cells were nearly absent from the inguinal LNs in the control animals.
We propose that activation of CD4 + T cells in response to FVIII requires a complex interaction of multiple types of APCs, which occupy distinct compartments in the secondary lymphoid organs that FVIII antigen traverses en route to cognate CD4 + T cells. In the spleen, where response to FVIII primarily takes place, antigens larger than 60 kDa (such as FVIII at 280 kDa) do not freely flow into the white pulp and instead need to be ferried by APCs. While DCs ultimately activate CD4 + T helper cells, MFs shuttle the antigen to DCs and may also provide innate activation immune signals.

Disclosures
No relevant conflicts of interest to declare.