Table 1

Major characteristics of the generic host response to silk fibroin, based on the approach of Williams (2008)89

CharacteristicsHost response
Protein adsorption and desorption characteristicsThe method of silk processing affects protein adsorption:
  • Serum adsorption increases with fibroin fibre hydrophobicity. Some immunoproteins show different adsorption; Bb and C1q complement factors only attach well to fibroin fibres, not the film, whereas IgG and C3 fragments adsorb on both.90

  • In contrast, when comparing different fibroin films, protein adsorption increased with hydrophilicity and the lowering of β-sheet content.91

Generalised cytotoxic effectsSilk fibroin shows generally low cytotoxicity:
  • The widespread use of silk as suture has demonstrated real-world evidence of minimal cytotoxicity.35

  • With silk films, no cytotoxicity was exhibited toward L929 cells or subcutaneously over 19 months in rats.92

  • Porous silk fibroin film placed into the rabbit cornea was gradually replaced by stromal tissue. Non-porous film had normal adjacent cells, even after 6 months.24

  • Spider silk peptides have low cytotoxicity to neural cells.71

Neutrophil activationNeutrophil activation is generally low:
  • Rat mucosal epithelial cells attracted a significantly lower number of inflammatory neutrophils when used with silk fibroin compared with a control; there was no local or systemic immunological incompatibility of fibroin.93

  • There was less neutrophil infiltration of wounds dressed with silk film than with a hydrocolloid dressing.94

Macrophage activation, foreign body giant cell production, granulation tissue formationMacrophage activation occurs, but infiltration is generally low. Granulation tissue formation occurs:
  • With mouse macrophage cells, insoluble fibroin particles induced significant tumour necrosis factor (TNF) release and activation.28

  • In mice, subcutaneous silk fibroin implantation produced macrophage migration inhibitory factor, but not other proinflammatory cytokines. There was no remarkable infiltration of macrophages or lymphocytes, although by day 180 some macrophages were present in the adjoining tissues.95

  • Silk fibroin induced a cellular response in which proinflammatory macrophages and multinucleated giant cells were associated with vascularisation. The silk fibroin induced granulation tissue formation.96

Fibroblast behaviour and fibrosisFibroblasts can grow directly on fibroin:
  • Fibroin annealing treatment affects early-stage fibroblast adhesion. By adjusting fibroin surface characteristics, cell repellent areas and cell spreading direction can be controlled.97

Microvascular changesSilk does not adversely affect microvasculature:
  • Silk fibroin fibres cocultured with human microcapillary endothelial and osteoblast cells produced a perfusable lumen containing red blood cells which anastomosed with host murine vasculature.98

Tissue/organ-specific cell responsesThere are few tissue/organ-specific cell responses, however the response can depend on material preparation and biocompatibility is both host and location dependent:
  • Silk fibroin used as an electrospun scaffold for bladder reconstruction in the rabbit, resulted in a mild acute and chronic inflammatory reaction,99 but inflammation did not occur when fibroin was used as a urethral sling in the rat.100

  • There was considerable inflammatory cell infiltration when used as a mesh in the vesicouterine space, although the acute inflammation lasted no more than 4 weeks.101

Activation of clotting cascadeThe extent of activation of the clotting cascade depends on multiple factors in processing:
  • Coating polyester with solubilised fibroin reduces thrombogenicity.

  • Fibroin thrombogenicity is reduced with methanol treatment.

  • Fibroin films bound lower levels of fibrinogen than did two synthetic polymers while the same amounts of adsorbed human plasma complement fragment C3 and IgG were detected.

  • Sulphation of silk fibroin can reduce the coagulant activity.

  • Multiple factors in fibroin preparation such as treatment temperature and solvent influenced the biological response, but silk-based vascular grafts were considered viable.102

Platelet adhesion, activation, aggregationPlatelet adhesion and activation can be controlled by silk preparation and blending:
  • Changing silk processing can affect platelet interaction,91 but multiple factors influence response.102

  • Chemicals or blending with other polymers can be used to decrease human platelet attachment. Heparin can be loaded as a drug into silk fibroin to reduce platelet attachment in vivo in pigs.

  • Conversely, in areas where platelet adhesion may be desirable such as wound sites, von Willebrand factor can be attached to fibroin.103

Complement activationComplement adsorption occurs with fibroin:
  • Fibroin preparation changes the extent of adsorption or activation with there being greater complement C3 fragment activation with films compared with fabrics.90 Binding may be specific to hydrophobic (β sheet) areas.

Antibody production, immune cell responsesSilk can produce an immune cell response, but removal of sericin may reduce this:
  • Silk can act as a potent allergen. Non-iatrogenic causes include type I allergic diseases, such as asthma, hypersensitivity pneumonitis and rhinitis.

  • In the human eye, severe reactions to virgin silk sutures used in cataract surgery have occurred suggesting a possible role of prior sensitisation.23

  • In the rabbit, porous fibroin is gradually replaced by stromal tissue without causing inflammatory reaction by CD4-positive and CD8-positive lymphocyte infiltration.24

Acute hypersensitivity/anaphylaxisSilk can cause hypersensitivity:
  • There has been anaphylaxis after eating the silkworm in its entirety.

  • In clothing, silk gives more positive reactions of eczema than any other protein.104

Delayed hypersensitivityDelayed hypersensitivity can occur:
  • A reaction to silk fibroin was reported with recurrent granulomas where the patient may have developed the reaction from silk sutures used in a previous surgery.27

  • Three paediatric neurosurgical patients had delayed inflammation 7 years following initial silk suture placement.26

Mutagenic responses, genotoxicityThere is no known evidence of a mutagenic or genotoxic response:
  • Silk fibroin genotoxicity has been tested in vitro using an Ames test employing a bacterial reverse mutation, and in vivo using a bone marrow micronucleus assay in the mouse. Genotoxicity was not evident.37

  • Indeed, silk fibroin peptides actively chemoprotective and reduced DNA damage during challenge of a mutagen on mouse embryos. The size and the glycine plus alanine ratio of peptides affected activity and the action was due both to the protective interactions between cells and peptide molecules and also to the direct inactivation of the mutagen.105

Reproductive toxicityThere is no known evidence of a reproductive toxicity aspect of fibroin or sericin:
  • A teratozoospermia ratio assay and morphological observation of testis histology in the mouse showed that there was no difference between silk fibroin solution and control.37

  • Indeed, in mice, silk fibroin amino acids improve male reproductive function by increasing testosterone and sperm count.

  • Silk hydrogels into the cervixes of pregnant rats showed only an initial mild foreign body response and there was no upregulation of inflammatory markers with in vitro human cervical cells.106 However, the longer-term impacts on pregnancy are unknown.

Tumour formationThere is no known evidence of tumour formation from fibroin or sericin:
  • Indeed, silk has been shown to have a protective effect against tumour promotion.33 34