The therapeutical approaches for rare diseases through the immune processes of IgG Fc Receptors

Fc receptors are a group of integral membrane glycoproteins molecules presented on phagocytic cell surface mainly and such specialised molecules specifi cally recognise and bind to the Fc portion of the immunoglobulins molecules after triggered by various cellular immune effector functions, to destroy and eliminate the opsonized target through the important roles in host defence and immune regulation in activation and inhibition (gain and loss functions) [1,2].


Introduction
Immunoglobulins or antibodies are the bi-functional molecules structurally featured with Fab (Fragment of Antigen Binding) and Fc (Fragment of Crystalised region) portions.
Citation: Wu  Fc type I (also known as CD64) is an integral cell membrane protein with a molecular weight of 68-72kDa found on monocyte, macrophages and myeloid leukemia cell lines activated [12]. Fc type I was fi rst isolated from the monocytic cell line (U937) and display high affi nity for monomeric IgG and showed preferential binding of IgG subclasses, in order of rank is IgG1=IgG3>IgG4; it does not react with IgG2 [13].
Fc type II (also known as CD32) is a 40kDa molecular weight glycoprotein which was initially isolated by affi nity chromatography of a monocytic cell line-U937 cell lysates on IgG-Sepharose in 1982 [13]. It was further characterised by a monoclonal antibody-IV. 3. Unlike Fc type I, Fc type II binds with low and medium affi nity for monomeric IgG [14,15].
Fc type III (also known as CD16) is a 50-70kDa protein with low affi nity distributed on neutrophils, macrophages and NK cells [10]. It was originally identifi ed by 3G8 (a monoclonal specifi cally against Fc type III) with Fc type IIIa and Fc type IIIb. Studies showed that FcRIIIb is an anchored molecule without the intracellular signalling motifs [16].
Rare disease is defi ned as a kind of diseases/disorders affecting a small population. It is estimated that there are more than 7,000 different types of diseases/disorders from the combination factors in the pathophysiological mechanism of genetics and environment or their interactions [17,18] The studies on Fcs have provided the understanding in disease mechanism it also provides the approaches for Rare disease therapies.

Characterisations of Fcγrs
Immune biological function of FcRs: FcRs are binding molecules on the surface of effector cells and soluble form in circulation reacting with the Fc part of the immunoglobulins or antibodies [19]. They play very important regulation roles in immune system between cellular and humoral immunity by enable antibodies to perform several biological functions, forming a link between specifi c antigen recognition and effector cells under the normal physiological conditions and the maintenance (homeostasis) of the balance in activation and inhibition in health control [20][21][22][23].
In the normal conditions, Fc portion of monomer IgG does not bind to Fc receptors, only if Fab binds to foreign or autoantigens or monomeric IgG aggregated and then it will trigger effector cells to destroy antibody-coated target cells through several immune processes such as the antibody-dependent cell mediated cytotoxicity (ADCC), Endocytosis, phagocytosis of immune complexes by effector cells, clearance of immune complexes, regulation of production of antibody by FcRs, biological substance release medicated by FcRs, and the regulation of cytokines mediated by FcRs [24][25][26][27][28][29].

The signifi cances of FcγRIIA on megakarocytic lineage of the platelets precursor cells
Fc type II is a transmembrane protein and a polymorphic molecule distributed widely on hematopoietic cells such as monocyte, macrophages, neutrophils, B-lymphocyte either as a sole type or in a combination with Fc type I or Fc type III with a variation of numbers of copies [15]. The presence of IgG Fc Receptors on platelets was demonstrated by Rosenfeld, et al. Such fi ndings showed platelets are cells with immune functions [14].
Further studies revealed that FcRIIA and FcRIIC protein has activation biological functions through the immunoreceptor tyrosine-based activation motif (ITAMa), and in contrast, FcRIIB protein has inhibition biological functions through the ITAMi [30]. Therefore, FcR type IIA,B and C have been extensive studied due to their important immune-biological functions, particularly in platelet activation associated diseases/disorders.
An interesting study demonstrated that IV.3, the monoclonal antibody specifi c again IgG Fc receptor type II inhibited platelet aggregation induced by the sera from patients with heparin induced thrombocytopenia indicating that heparin induced thrombocytopenia is mediated by IgG Fc receptor mechanism [31].
Platelets are differentiated from its precursor cellsmegakaryoblasts matured to megakaryocytes. It was presumable that Up-regulation of FcRs on or in platelet could be occurred at megakaryocyte/megakaryoblast level under the infl uence of cytokines which it would further exacerbate the pathological processes. Chong's group have characterised FcRII on human megakaryocytes isolated from bone marrow by using Magnetic Activated Cell Sorting (MACS), megakaryoblasts and megakaryoblastic cell lines (MEG-01 and UT-7) and by using a combination of techniques including immune-cytochemical staining, dual colour fl ow cytometry, immunoprecipitation from the protein level. Finally they studied mRNA expression of FcR type II and their transcripts. Their results showed the presence of FcR type IIA gene which is the Transmembrane (TM) exon [32][33][34][35][36][37].
Interestingly, they also found the presence of FcR type IIA from protein to mRNA level on the early haematopoietic progenitor cell stages, stem cell antigen (CD34)-expressing cell line (KG-1) and its less differentiated subline (KG-1a) [38] and as summerised in Table 1.
The characterisation of FcRIIA on cells of megakaryocytic lineage at protein and RNA levels enhanced the understanding of the pathophysiology in the FcRs regulation of the platelet activation associated diseases/disorders from the protein to RNA levels.
The signifi cances of these studies were two fold; the fi rst was the identifi cation of the presence of FcR type IIA on platelet and their precursor cells, megakaryocytes and megakaryoblasts from protein to molecular level. The transmembrane molecular of FcR type IIA found on these cells have implied the pathophyological mechanism in some platelet activation associated diseases/disorders because FcR type IIA may play a physiological role from excessive binding and injurious effects of immune complexes such as in immune thrombocytopenia.
The second signifi cance was that the sole type of the FcRIIA identifi ed on these cells (megakaryocytes, megakaryoblasts and the CD34 expressing cell lines, KG-1 and KG-1a can be used as an identifi cation marker in hematopoietic lineage differentiation stage.

Genetic variation of FcγRs
Multiple genetic variations including Single Nucleotide Polymorphisms (SNPs) and Copy Number Variants (CNVs) on low and medium affi nity have been identifi ed in the FCGR loci [39][40]. Studies revealed that Fcs are composed of homologous immunoglobulin-like Extracellular (EC) domains, divergent Transmembrane (TM) and Intracytoplasmic (IC) regions with different degree of heterogeneity on different cells with different coding gene variations [41][42].
Three very similar genes (A, B and C) have been characterized for Fc type I located on chromosome 1q21.2 by the gene cloning studies [43][44][45][46]. Six variants transcripts of the FcR type I were further identifi ed [47].
Three types of the FcRII (A,B and C) with low and medium affi nity identifi ed they are encoded with the mRNA splice variants of FCGR2A, FCGR2B and the A and B recombination variant-FCGR2C of these three gene products with conserved extracellular domains located on chromosome 1q23.3 on the FCGR2/3 locus [48].
FcRIII also is a low and medium affi nity receptor with A and B genes (FcRIIIA and FcRIIIB) identifi ed also located on chromosome 1q23.3 on the FCGR2/3 locus [46,49].
Studies showed such genetic variations affecting immune functions, increasing individual's susceptibility and impacting therapeutical response of disorders/diseases [50][51][52]. Studies also showed that there are many different kinds of disorders/ disease linked with such genetic variations associated dysfunctions of FcRs (gain or loss) with different consequences occurring with heterogeneity among different ethnic groups [53][54][55].  KG-1A + [38] Citation: Wu  (CLL) and they found that FcR expression was a bad prognostic factor, independent of age and sex, but correlated well with the tumor mass score [78]. However, Dornan, et al. results suggested that FCGR2A and FCGR3A polymorphisms did not signifi cantly change the outcomes of relapsed or refractory CLL patients treated with fl udarabine or the monoclonal antibody [79].

Therapeutic trials through the immune processes of FcγRs on rare diseases
Advancing studies on genetic variations on FcRs increased the understanding of disease mechanism and translated such fi ndings into clinical applications, such as in disease monitoring of therapeutic response and disease prognosis.
The therapeutic trials with monoclonal antibody in immune disorders were reported [80]. Bio-therapeutical monoclonal antibody against FcR by the engineering approaches showed it is an effective through the immune processes of FcR for many different kinds of diseases clinically [81].
Intravenous gamma immunoglobulin (IVIG) therapy has been used to treat some kinds of immune mediated rare diseases successfully for several mechanisms, One of the application mechanisms of such therapy in such diseases is to block, or to compete the binding with immune complexes on effector cells, to mediate Fc receptor on effector cell surface to regulate immune responses, particularly in immune thrombocytopenic purpure [82,83].
An infusion of 3G8 (a monoclonal antibody specifi c against FcRIII) was given to a patient with refractory idiopathic thrombocytopenia purpure. It resulted in a dramatic rise the platelet count, which reached normal level for two weeks [84].
The clinical effect was assumed to have been brought about, in part by a modulation of mononuclear phagocytic function, particularly by inhibiting of FcR-mediated phagocytosis.

FcγRs and coronavirus disease 2019
Beyond the rare diseases reviewed above, the roles of immune regulations and the therapeutical approaches for Citation: Wu  Studies showed the infection of SARS-CoV-2 enter host cells through the receptor of Angiotensin-Converting Enzyme (ACE2) [85,86]. In addition, Studies from Takeda, et al. Immunotherapy with immune IgG collected from patients recovered COVID-19 combined with antiviral drugs could be an alternative treatment against COVID-19 although some clinical practical issues need to be solved and procedures need to be improved [87,88].
No doubt, further studies will be conducted in the future to investigate the possibility of therapeutical approaches for COVID-19 such as to block and neutralise viral activity and to regulate the cytokine storm though the immune processes and the genetic variation through FcR can be used as one of the treatments. As other diseases, genetic variation is associated with the susceptibility, clinical severity and mortality of the COVID-19. It has been noted that there were some COVID-19 cases reported to be reinfected in couple of month time.
In addition, thrombocytopenia is one of the clinical features of COVID-19 and it is reported to be associated to clinical severity of disease [89]. There are several possibilities resulting in thrombocytopenia including increase destruction of platelets due to the coagulation status, decreased production of platelets due to therapies. One of the mechanisms of thrombocytopenia could be due to immune mechanism mediated by FcRs.

Conclusions
Human immune includes cellular immune and humoral immune. Fc receptors play very important role by linking cellular and humoral immune in the homeostasis, regulation and host defence in the host defence and homeostasis in both of the adaptive and innate immune system. FcR is the major type of the fi ve Fc receptor classes found to be distributed on different types of cells. Many immune diseases such as autoimmune disease, immune defi ciencies and even cancers can be mediated by the dysregulated of FcR.
The characterisation on FcRs from protein to molecular level has been intensively studied, particularly in their genetic variations associated with disease susceptibilities.
Advancing studies on FcRs have revealed mysteries of many different kinds of diseases in the understanding of the disease pathogenesis for clinical settings. Recently, the interests of therapeutical approaches for diseases through the immune processes of FcRs have been drawn.