Immunological Aspects and Immunomodulation

and Veronica Tomasello2

(1)
Department of Plastic Surgery and Burns, University Hospital Vall d’Hebron, Barcelona, Spain
(2)
Cannizzaro Hospital, Catania, Italy
 
Abstract
Vascularised composite tissue allotransplantation (VCA) is a new transplant discipline of reconstructive plastic surgery that relies on the restoration of deformity and/or amputation by the allotransplantation of different tissues and organs from another person (the donor). As in any other solid organ transplant discipline, graft survival depends on the acceptance of transplanted tissues and organs on the recipient and the absence or control of rejection episodes. In 1997, the Louisville team proved that long-term survival of transplanted limbs could be achieved in an experimental model with a triple-drug therapy (steroid, tacrolimus and mycophenolate mofetil). Soon afterwards, the first human hand transplants were a reality in 1998. The first experiences received the same immunosuppression regime, with excellent long-term results (both functional and immunological) to date. The current success of VCA is a promising field in plastic and reconstructive surgery. Intense research is currently being undertaken to improve the immune tolerance of composite tissues, since long-term results and face or limb acceptance depend on immune regulation and the control of side effects.
Keywords
ImmunosuppressionComplicationsRejection

11.1 Introduction

Vascularised composite tissue allotransplantation (VCA) is a new transplant discipline of reconstructive plastic surgery that relies on the restoration of deformity and/or amputation by the allotransplantation of different tissues and organs from another person (the donor). As in any other solid organ transplant discipline, graft survival depends on the acceptance of transplanted tissues and organs on the recipient and the absence or control of rejection episodes. In 1997, the Louisville team proved that long-term survival of transplanted limbs could be achieved in an experimental model with a triple-drug therapy (steroid, tacrolimus and mycophenolate mofetil). Soon afterwards, the first human hand transplants were a reality in 1998. The first experiences received the same immunosuppression regime, with excellent long-term results (both functional and immunological) to date. The current success of VCA is a promising field in plastic and reconstructive surgery. Intense research is currently being undertaken to improve the immune tolerance of composite tissues, since long-term results and face or limb acceptance depend on immune regulation and the control of side effects.
Face VCA implies the transplantation of diverse tissues, such as skin, muscles, bones, lymph nodes, nerves, blood vessels, cartilage, soft tissues, salivary glands and mucosa. Given the nature of all these tissues, it is very difficult to parallel the immunological results of face VCA to that of solid organ transplantation. Face VCA is composed of different tissues with diverse types of immunogenicity and immune response. There is some consensus on the general type of immunosuppression regime that is necessary in VCA, although the perfect drug combination is still to be defined. However, functional outcome depends much on tissue survival and the immunological response; thus immunomodulation during induction, immediate postoperative period and long-term follow-up is very relevant for the final outcome and the improvement in quality of life.

11.2 Immunological Aspects of VCA Tissues

11.2.1 Skin

The cutaneous component of face VCA is an important part of the face graft. It is what we see, how we feel, the interaction with society and the basis for expression. Its colour, quality, structure and texture indicate to others our emotions, age, temper and health.
The skin is a highly immunogenic tissue with active immunological functions. Apart from the structural layers (epidermis and dermis), the skin has resident cellular elements and cells in transit with immunological and pro-inflammatory properties. This particular immunological microenvironment is termed skin-associated lymphoid tissues (SALT). Cells that participate in this system are diverse (Table 11.1); the most important part of it includes antigen-presenting cells (APC). This microenvironment is responsible to drain to the regional lymph nodes and report its status. It is very relevant in transplant physiology for up and down immunological regulation and for the development of short- and long-term tolerance. The functions of cutaneous antigen-presenting cells include:
Table 11.1
Skin-associated lymphoid tissues (SALT)
1. Antigen-presenting cells (APCs):
 Langerhans cells
 Dendritic cells
2. Lymphocytes
3. Keratinocytes
4. Fibroblasts
5. Endothelial cells
6. Local/regional lymph nodes
  • Expression of high levels of class I and II antigens of the major histocompatibility complex (MHC)
  • Expression of CD80 and CD86 (molecules for co-stimulation)
  • Internalisation and process of antigens
  • High capacity of migration (allows for antigen transportation)
  • Stimulation of allogenic T cells
Cutaneous APCs (Langerhans cells and dendritic cells) are normally inactive in a stationary status; however, a few numbers of cells migrate to regional lymph nodes to present antigens on a regular basis, which allows for a status of tolerance. Under certain circumstances (i.e. inflammatory stimulus), APCs respond in an intense manner; this response allows for a correct reaction against the insult, whereas the slow-rate migration maintains the tolerance against self antigens.
Other immunological skin resident cells include T cells, which are normally present within the dermis. They are represented by CD4 cells (T-helper subpopulation) and CD8 (T-suppressor and cytotoxic). T-helper cells may be subdivided into Th1, memory cells responsible for the initiation of the cell-mediated response (they secrete pro-inflammatory cytokines IL-2, IL-12 and IFN-γ) and Th2 (they produce IL-4, IL-5, IL-6, IL-10 and IL-13) responsible for B-cell response.
The immunological response can also be started by keratinocytes. They are a source of pro-inflammatory cytokines IL-1, IL-6, IL-8 and TNF-α; they have a systemic effect on the immune system, modulate the proliferation of keratinocytes and attract inflammatory cells. IL-7 and IL-15 contribute to the circulation of T cells; and IL-10, IL-12 and IL-18 are responsible for the systemic response. The stimulation of keratinocytes with IFN-g produces the expression of MHC class II proteins and intracellular adhesion molecule I (ICAM-I), which explains that keratinocytes play a role both as APC and in the induction of functional immune reaction.

11.2.2 Regional Lymph Nodes

Transplantation of composite tissues carries with it a certain amount of lymphatic tissue, mainly in the regional drainage lymph nodes and in the bone marrow should the VCA include bone. Nodes are a source of immunocompetent T cells, B cells and follicular dendritic cells. They contribute to the immunological response of the transplant on the recipient. Competent recipient’s T cells migrate to the regional lymph node of the transplant and they proliferate. When immunosuppression drug levels are inadequate, they may trigger an acute rejection episode. On the other hand, cutaneous dendritic cells may migrate into the regional lymph node and trigger a secondary immunological response when in contact with the recipient’s T cells.

11.2.3 Muscle

Face transplantation is intended not only as a restorative operation but also a quality improvement therapy. Face muscles play an important role in quality improvement, especially face sphincters (oral and orbital), which restore important functions. An absent inflammatory and immune response in the muscular layer is necessary to maintain a good functional outcome. Under physiological conditions, muscle cells do not express MHC class I or II molecules (they do express class II molecules in in vitro culture of muscles cells stimulated with IFN-a, IFN-g and IL-1b). Hence, we may hypothesise that muscle cells may express class II molecules in certain viral and bacterial infections, present antigens and produce autoantigens. On the other hand, skeletal muscle expresses HLA-G, a MHC class I molecule that is involved in immune tolerance.

11.2.4 Peripheral Nerves

Correct reinnervation of the transplanted face is responsible for an adequate sensation and motor function of face muscles. Peripheral nerves include different cell types and tissues: neuronal axons, Schwann cells and connective tissue. Connective tissues and cellular elements of the immune system are present in the epineurium and perineurium. Schwann cells may produce different pro-inflammatory cytokines (IL-1, IL-6, TNF-α), other mediators (prostaglandin E, thromboxane A, leukotriene C) and immune modulators (TGF-β). Schwann cells express MHC class I antigens in normal circumstances. Following nerve injury, they express class II antigens and IFN-g in the presence of activated T cells. This suggests that Schwann cells may function as APC (antigen-presenting cells) and play a role in local immune response. Immunomodulation is mediated through the production of erythropoietin. It prevents axonal degeneration, reduces the production of TNF-α, prevents Wallerian degeneration and diminishes neuropathic pain. Transient activated T cells and B cells can be located in peripheral nerves, independently from antigen specificity. APCs are normally represented by local macrophages, expressing MHC class II antigens and co-stimulating molecules B7-1 and B7-2, which are essential in T cells for antigen presentation and local immune response.

11.2.5 Bone and Bone Marrow

Face bones are an integral part of face transplantation when the mandible and/or the maxilla are reconstructed. Immune bone antigenicity is considered to be low. However, limb and face VCA may include bone marrow, which carries with it a significant amount of haematolymphoid cells. In adequate conditions, the transplantation of bone marrow cells in non-myeloablative immunosuppression creates a status of immunomodulation and downregulation of the recipient’s immune system and can be involved in the induction of tolerance. Following limb or face VCA that includes bone marrow (or in research protocols for tolerance induction with infusion of donor’s bone marrow), haematopoietic and donor’s lymph cells migrate (including dendritic cells and APCs) from transplant tissues and colonise lymphoid and non-lymphoid recipient’s organs (chimerism). When these migrating cells colonise and react against the recipient’s tissues, graft-versus-host disease may develop, which may be fatal.

11.2.6 Blood Vessels

Face transplantation follows the same principles of reconstructive microsurgery. In general terms, the face graft can be conceived as a free flap prefabricated by nature and harvested from another human being. Correct circulation with enough inflow and adequate outflow is necessary for flap survival. Endothelial cells play a multifactorial function: regulation of haemostasis, cell adhesion, vasomotor tone regulation and immune regulation. The latter is mediated by leucocyte and other immune cells’ adhesion and migration across the endothelium. In inflammatory conditions, endothelial cells produce IL-1, IL-6 and IL-8 and activate the expression of P selectin, E selectin and cellular adhesion molecules (ICAM-1, ICAM-2 and VCAM-1), which facilitate leucocyte migration. They also facilitate the proliferation and differentiation of activated T cells and Th memory cells through APC, MHC class II molecules and CD40 co-stimulating molecule.

11.2.7 Salivary Glands and Oral Mucosa

Salivary gland immune function is mediated through the mucosal cells and the associated lymphoid system. Acinar and ductal cells excrete glycoproteins for IgA and IgM. Lymphoid system is located in a diffuse manner within the glands and in regional lymph nodes. This tissue is an important effector site for the immune reactions of the oral mucosa and regulates the cellular and antibody immune reactions. Flow cytometry differentiates mononuclear cells of T, B and NK origin.
Oral mucosa is an integral part of most face transplants. Its physiologic function includes the recognition and elimination of pathogens and tolerance of commensal bacteria necessary for immune homeostasis. The most important cells of mucosal immune reactions are dendritic cells. They express CD1a molecules for the presentation, activation and maturation. Other identified cells in oral mucosa include dermal and plasmacytoid dendritic cells which contribute to the pool of CD83+ dendritic cells of the basal lamina and to viral antigen presentation. Other immune elements of oral mucosa include lymphoid and myeloid cells, such as basal lamina cells, dendritic cells and CD4+ T cells with expression of CD45RA and CD45RO.

11.3 Immunological Monitoring

11.3.1 Pre-transplant Evaluation

The monitoring and immunological outcome of face transplants depend on risk factors that should be evaluated in the overall candidate evaluation such as the determination of donor-specific preformed antibodies (DSA). During the overall evaluation in the pre-transplant phase, tissue typing (HLA), anti-HLA screening and compatibility tests are performed. The experience in other solid organ transplantation indicates that high levels of DSA should be avoided to reduce the risk of hyperacute rejection.
HLA typing has been traditionally performed by serological methodology. However, recent work indicates that most laboratories are currently utilising molecular tests for tissue typing, including primer-specific PCR sequencing (sequence-specific primer [SSP]), specific sequencing of oligonucleotide primer (SSOP) and direct DNA sequencing. Typification of donor and receptor HLA allows:
Apr 2, 2016 | Posted by in General Surgery | Comments Off on Immunological Aspects and Immunomodulation

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