Background/Purpose: SLE is a
multisystem autoimmune disease that displays quantitative and qualitative
deficiencies of regulatory T cells (Treg), notably because of a compromised
IL-2 production. Signaling lymphocyte activator molecule receptor family 3 (SLAMF3)
is a type I transmembrane receptor implicated in co-regulatory mechanism of T
cell activation. We
examined the effects of SLAMF3 mediated co-stimulation on the IL-2 response of
peripheral CD4⁺ T cells from SLE patients.

Methods: T cells or naive CD4+ T cells isolated
from SLE patients and matched controls were stimulated with plate bound anti-CD3,
anti-SLAMF3 and/or anti-CD28 antibodies. Cells were then stained for surface
markers (CD4, CD8, CCR7, CD45RA, CD25, CD122, CD127, SLAMF3), for intracellular
cytokines (IL-2) and/or transcription factors (pSTAT5, FoxP3) and analyzed by flow
cytometry. CD25 mRNA levels were evaluated by qPCR, and pSTAT5/STAT5 ratio by
western immunoblots. Cells proliferation was determined by CFSE dilution. Treg differentiation
was performed on naive CD4⁺ T cells cultured under Treg polarizing
conditions (in the presence of IL-2 and TGFb) for 6
days. Suppression capacity was determined by assessing the proliferation of
autologous CFSE-labeled T cells in the presence of induced Treg.

Results: Stimulation of naive
CD4⁺ T cells with anti-CD3/anti-SLAMF3 upregulated surface CD25
(IL-2Ra) and CD122 (IL-2Rb), at higher level compared to
anti-CD3/anti-CD28 activation. Anti-CD3/anti-SLAMF3 co-stimulation of naive CD4⁺
T cells led to increased phospho-STAT5 (pSTAT5) levels compared to anti-CD3/anti-CD28
activation. Although naive CD4⁺ T cells from SLE patients displayed
decreased pSTAT5 levels compared to control subjects upon anti-CD3 or
anti-CD3/anti-CD28 stimulation, co-stimulation of SLE T cells with anti-SLAMF3
restored pSTAT5 to a normal level. The
effect of SLAMF3 on the IL-2/IL-2R/STAT5 signaling pathway does not occur
through increased IL-2 production, as the percentage of IL-2 producing cells
remained comparable between anti-CD3 and anti-CD3/anti-SLAMF3 mediated
co-stimulation. Exogenous IL-2 increased proliferation of
anti-CD3/anti-SLAMF3-activated CD4⁺ T cells compared to
anti-CD3/anti-CD28 stimulation. Naive CD4⁺ T cells activated with
anti-CD3/anti-SLAMF3 under Treg polarizing condition expressed high levels of
FoxP3 and CD25, low level of CD127, and, more importantly, exhibit a potent
suppressive effect.

Conclusion: SLAMF3-mediated
co-stimulation enhances naive CD4⁺ T cell response to IL-2 in SLE patients
and promotes Treg generation. These results suggest that using a monoclonal
antibody directed against SLAMF3 may increase the response to IL-2 in
conditions where its availability is compromised, like SLE.

Figure 1. SLAMF3 mediated
co-stimulation promotes the IL-2 response of naive CD4+ T cells