*6.2. Protein Kinase C δ Deficiency*

Mutation impairing the protein kinase C δ (PKCδ) structure or function leads to a clinical phenotype featured by an increased risk of infections with hypogammaglobulinemia, autoimmunity, and lymphoproliferation [104]. PKCδ is implicated in the activation of different transcriptional factors with a central role in the immune homeostasis of B lymphocytes [105]. It is activated in the context of a wide number of molecular pathways, including mTOR, PI3K, and mediates the transcription of STAT1 (leading to enhanced

IFN-stimulated transcription), ERK1, and other factors. PKCδ induces also the transcription of the IL-10 gene and reduces the production of IL-6. Finally, it promotes apoptosis through the interaction with caspase 3, thus limiting lymphocyte proliferation [104,106]. In PKCD, autoimmunity depends on the accumulation of self-reactive lymphocytes, the production of different autoantibodies, and defects in the process of lymphocyte negative selection [105]. The most common manifestations of autoimmunity are represented by arthritis, cytopenia, and SLE-like manifestations, such as glomerulonephritis and photosensitive rash [104]. Moreover, the ineffective control of apoptosis causes lymphoproliferation, and some patients presented with a clinical phenotype mimicking autoimmune lymphoproliferative syndrome (ALPS) [107]. As literature reports only a small number of cases of patients diagnosed with PKCD, the therapeutic approach has yet to be defined. However, available data suggest the use of conventional immunosuppressive strategies approved for SLE, including hydroxychloroquine, mycophenolate mofetil, and rituximab, and, when the lymphoproliferative aspect is prominent, the adoption of anti-mTOR drugs, such as sirolimus [104]. The administration of tocilizumab in patients with high IL-6 serum levels and the use of HSCT represent promising therapeutic strategies [104].

Interestingly, a similar clinical picture can be observed in the RAS-associated autoimmune leukoproliferative disease (RALD), a disease caused by mutations in the RAS signaling pathway (often NRAS or KRAS), which is currently classified among the phenocopies of PIDs. In RALD, uncontrolled activation of RAS-dependent molecular signaling is responsible for the development of hepatosplenomegaly, lymphadenopathy, autoimmune cytopenia, and SLE-like manifestations [108].

#### **7. Autoimmunity in Disorders of Cytoskeletal Function**

Cytoskeletal proteins are essential for multiple cellular functions, including the immunologic synapsis between T cells and APCs, and the regulation of lymphocyte proliferation.

Wiskott-Aldrich syndrome (WAS) is an inherited condition caused by mutations in the WASP gene on the X chromosome, which encodes the WAS protein, an actin-nucleation promoting factor expressed in hematopoietic stem cells [109]. WAS is clinically featured by a classic triad of thrombocytopenia with small-size platelets, eczema, and lymphopenia, mainly affecting T cells [110]. The molecular defect influences multiple cellular lineages and causes complex implications on the immune function, including ineffective T cell proliferation and function, reduced Treg activity (with preserved absolute Treg values), and hyperproliferation of B cells, which show enhanced production of autoantibodies [110]. The disease severity of WAS patients is variable, and the more commonly reported autoimmune manifestations are AIHA, autoimmune neutropenia, peripheral vasculitis, and arthritis [111]. Although patients with a clinical picture dominated by thrombocytopenia could benefit from splenectomy, this intervention does not reduce the risk of autoimmunity. Consequently, the definitive treatment of patients with WAS is currently represented by HSCT or gene therapy [111].

Another disease featured by altered cytoskeletal structure and function is DOCK8 deficiency, which shares some common clinical features with WAS. In this condition, the molecular defect is responsible for an altered cytoskeletal actin regulation, which causes reduced proliferation, migration, and function of innate and adaptive immune cells, and impaired Treg activity [112].

Clinically, patients with DOCK8 deficiency show a picture of combined immunodeficiency featured by recurrent cutaneous and respiratory infections, eczema, increased risk of malignancies, and predisposition to the development of atopy (with high IgE levels), and autoimmunity (cytopenia, thyroiditis, vasculitis, uveitis). In absence of specific gene therapy, the only curative treatment for DOCK8 deficiency is HSCT [111,112].

#### **8. Autoimmunity in Complement Deficiencies and Disorders of Innate Immunity**

Genetic defects causing deficiency of components of the complement cascade are associated with an increased risk of bacterial infections caused by capsulate agents. This derives

from the pivotal role of the classic and alternate complement pathways in determining bacterial lysis. However, deficiencies of specific complement proteins (C1q, C1r/s, C2, C4a, C4b) also represent a risk factor for the development of different autoimmune manifestations, such as SLE-like features, glomerulonephritis, JIA, and dermatomyositis [2,113,114]. The pathogenic mechanism linking complement deficiencies with autoimmunity is mostly dependent on a reduced clearance of apoptotic cells and immune complexes. This causes an enhanced availability of auto-antigens, which are chronically exposed to the immune system, thus triggering a self-reactive immune response [114,115]. Moreover, complement deficiency can impair B-cell negative selection, thus allowing the expansion of self-reactive B-lymphocytes, and can contribute to the development of autoimmunity [113]. Since in patients with complement deficiency the immunization against capsulated bacteria significantly improves the outcome, recognizing a patient with this condition and characterizing the specific defect is essential to improve the long-term management, including the anti-infectious prophylaxis.

Concerning disorders of innate immunity, the incidence of autoimmune manifestations is lower compared to PIDs affecting the adaptive response. However, in patients with chronic granulomatous disease (CGD), a higher incidence of SLE-like clinical features, arthritis, and other autoimmune conditions (hepatitis, nephritis) is observed [116]. Although the pathogenesis of these manifestations is not completely elucidated, it is accepted that the process involves the persistence of infectious antigens, inflammasome overactivation, altered production of neutrophil extracellular traps, and defective apoptosis [2,117]. The treatment of autoimmunity in CGD is challenging, since the need to use steroids and immunosuppressive agents should be balanced with the high infectious risk observed in this population. Currently, HSCT and gene therapy are the only curative treatments for CGD [116,117].

## **9. From Theory to Bedside**

The above-discussed pathogenic and clinical associations carry different significant implications for the approach to children with autoimmunity, highlighting that it should represent a warning sign for the presence of a PID, particularly in pediatric age. Identifying a condition of PID in children presenting with autoimmunity offers the opportunity to provide an adequate treatment of the underlying disease (Ig replacement therapy, targeted treatments, HSCT, gene therapy), to offer supplementary immunization if needed, and optimize the management of autoimmunity itself (Figure 2). On the other hand, as different studies have demonstrated a reduced survival in the subgroup of patients with PIDs showing autoimmunity, its finding in patients with already diagnosed PID could lead to significant changes in the follow-up strategy and therapeutic approach [118,119].
