Sickle Cell Disease Patients Have Altered Number and Function of Dendritic Cells

Dendritic cells (DCs) are the sentinels of the immune system able to recognize pathogen- and damage-associated molecular patterns (PAMPs and DAMPs), promoting a bridge between the innate and adaptive immune systems. They form a heterogeneous group of cells with different development, phenotype and functions, and they are mainly classified in conventional DCs 1 (cDC1) and 2 (cDC2), plasmacytoid DCs (pDC) and inflammatory DCs. Changes in the development of DCs, in the ratio of the subsets or in the maturation and activation can impair immunity or tolerance, inducing susceptibility to infections, as well as the development of inflammatory and autoimmune diseases. Sickle cell disease (SCD), one of the most common hemoglobinophaties throughout the world, can be considered as a chronic inflammatory disease, with systemic release of TNF-α, IL-6, IL-1β and IL-8. Despite presenting more inflammation, SCD patients have dysfunction of lymphocytes T and B responses, and are more susceptible to infections. Although DCs are the main responsible for the activation and polarization of lymphocytes function and are able to produce the pro-inflammatory cytokines present in the serum of SCD patients, very little attention has been giving to these cells so far.

In the present study, we characterized the subpopulations of circulating DCs in SCD patients. Our data of flow cytometry show that SCD patients (SS) have higher percentage (AA: 0.6±0.09, n=20; SS: 1.25±0.2 n=22; p<0.05) and total number/μl (AA: 10.92±1.9, n=17; SS: 27.9±5.2, n=22; p=0.01) of circulating DCs than healthy individuals (AA) as shown by gating on linage^- (CD19/CD56/CD3/CD14) and HLA-DR⁺ cells. The development of these cells from the bone marrow is likely to be, at least in part, responsible for this raise, since the percentage (AA: 1.7±0.45, n=17; SS: 10.85±1.53, n=11; p=0.0001) and total number/μl (AA: 0.25±0.05, n=12; SS 1.52±0.31, n=16; p=0.0004) of the DCs precursors are increased in the blood of the patients. The ratio of the different subpopulations of DCs is also altered, with a decrease in the percentage of circulating cDC1 (linage^-HLA-DR⁺CD141^high) (AA: 2.6±0.2, n=16; SS: 2.0±0.3, n=18; p=0.02), whereas no change was seen on cDC2 (linage^-HLA-DR⁺CD1c⁺) and pDCs (linage^-HLA-DR⁺CD123⁺) (AA: n=18; SS: n=22).

Isolated circulating monocytes from patients are able to differentiate in vitro in DCs with the expression of the characteristic markers CD1c and CD209, however, unlike healthy individuals, a great percentage of these cells also express CD14 (AA: 5.5±2.2, n=16; SS: 17.6±6.1, n=14; p=0.02), indicating the presence of inflammatory DCs. These cells are more mature and activated in steady state as shown by CD83 (p=0.007), CD86 (p=0.0001) and HLA-DR (p<0.0001) expression (n=14 pairs of CD14^- and CD14⁺ DCs). Since SCD patients have higher number of circulating monocytes as shown by previous studies, it is possible that part of the raise in circulating DCs comes from the differentiation of inflammatory monocytes in vivo.

Monocyte-derived DCs (mo-DCs) from patients are able to induce more proliferation of CD4⁺ and CD8⁺ T lymphocytes than mo-DCs from healthy individuals (AA: n=4; SS: n=4), and CD14⁺ DCs induce higher proliferation of T lymphocytes than CD14^- DCs from the same patient (n=2 pairs of CD14^- and CD14⁺ DCs). We have also seen that SCD patients have reduced percentage of regulatory T cells, as shown by Foxp3⁺ in the gate of CD3⁺CD4⁺ cells (AA: 4.0±0.26, n=17; SS: 3.1±0.3, n=17; p=0.01), and increased IL-17-producing CD4⁺ (AA: 1.1±0.08, n=18; SS: 1.7±0.18, n=17; p=0.05) and CD8⁺ (AA: 0.24±0.02; SS: 0.68±0.1, n=17; p=0.0001) T lymphocytes in the blood. Whether DCs are responsible for this skewing of T lymphocyte phenotype in SCD is still to be determined.

Our data so far show that there are differences in circulating DC populations in SCD patients and they could explain the changes observed on T cell responses and the susceptibility to infections. The present data add new knowledge about the chronic inflammation in SCD, which is one of the major events in the pathophysiology of the disease.