Etiology and genetics of collagen diseases

Functional analyses of the eyelid skin constituting the most soft and smooth area on the face: contribution of its remarkably large superficial corneocytes to effective water-holding capacity of the stratum corneum

BACKGROUND/PURPOSE

The eyelid constitutes a unique area on the face because of its soft, smooth and thin skin distinct from that of other facial portions. Its softness facilitates their easy compliance to blinking movement, which is indispensable to protect the wet surface of the eyeball. Moreover, the skin of the eyelid does not show any prominent follicular orifices or an oily appearance even in adults. Despite such uniqueness, its biophysical characteristics have remained unclear as compared with other facial skin.

METHODS

We conducted non-invasive instrumental measurements on the skin of the upper eyelid of 22 healthy Japanese adults in comparison with those of the adjacent facial skin, i.e. the cheek and nose. Additionally, we examined 10 adult patients with atopic dermatitis (AD) whose facial skin remained clinically free from skin lesions for at least 2 months.

RESULTS

The eyelid skin showed high transepidermal water loss like other facial skin. Its skin surface hydration state was as high as that of the adjacent skin, despite the fact that the eyelid skin revealed extremely low amounts of surface lipids unlike its neighboring skin regions. However, in contrast to small corneocytes found in other facial areas, the corneocytes of the eyelid skin displayed a significantly larger surface size, suggesting that slow turnover of its stratum corneum (SC) takes place to allow sufficient maturation of the corneocytes, enabling them to exert efficient water-binding capacity. Its pH tended to be higher than that of the adjacent skin. Its superficial blood flow was significantly higher than that of others, although skin color assessment showed lower color values for redness than those of the other facial skin sites. In clinically non-lesional skin of adult AD patients, we found increased blood circulation and a higher parameter for redness, suggesting the presence of invisible mild inflammation in the dermis even long after subsidence of visible inflammatory changes.

CONCLUSION

Our present biophysical findings suggest that, although the eyelid skin is poor in surface lipids, its extraordinary large superficial corneocytes play an important role in maintaining sufficient hydration state of its skin surface to keep it soft and flexible, enabling its good compliance with the blinking movement.

The second and third extracellular domains of FcgammaRI (CD64) confer the unique high affinity binding of IgG2a

FcgammaRI (CD64) is functionally unique as it is the only FcgammaR able to bind monomeric IgG with high affinity. FcgammaRI is also structurally distinct, containing an extracellular Ig-interactive region of three Ig-like domains in contrast to the two domains of the low affinity receptors FcgammaRII and FcgammaRIII. Previous studies have demonstrated that the third domain of FcgammaRI plays a crucial role in high affinity IgG binding of the receptor, with the first and second domains together forming a low affinity IgG binding motif. In this study the individual functional contributions of the first and second domains of FcgammaRI to IgG binding have been investigated. Chimeric FcgammaR were generated by exchanging extracellular domains between mouse FcgammaRI and the structurally related yet distinct low affinity receptor for IgG, mouse FcgammaRII. The replacement of both domains 1 and 2 of FcgammaRI with domains 1 and 2 of FcgammaRII results in a dramatic change in IgG binding characteristics, as this receptor loses the capacity to bind monomeric IgG with high affinity and also demonstrates a broader specificity (binding not only IgG2a but also IgG1 and 2b. IgG3 was not tested). However, the substitution of FcgammaRII domain 2 of this chimeric receptor with domain 2 of FcgammaRI (generating a chimeric receptor with domain 1 of FcgammaRII linked to domains 2 and 3 of FcgammaRI) was found to reconstitute the specific high affinity monomeric IgG2a binding of wild-type FcgammaRI, albeit with a slightly reduced affinity (1.8-fold lower than wild-type FcgammaRI). These findings suggest that it is the specific interaction between domains 2 and 3 of FcgammaRI, with domain 1 playing a supporting role in maintaining the conformational stability of the receptor, that is the major structural requirement to confer the unique Ig binding characteristics of FcgammaRI.

New environmental agents associated with lupus-like disorders

An increasing number of environmental agents are being investigated as possible risk factors in the etiology of certain connective tissue disorders. Exposure to a variety of therapeutic agents, foods and dietary supplements, occupational and other toxic exposures, and infectious agents has been associated with the onset of lupus-like disorders. The mechanisms by which these agents might induce lupus remain unknown but may involve alteration of cellular components or activation of the immune system. Individual host susceptibility factors, including pre-existing organ dysfunction and particular metabolic enzyme or immunogenetic phenotypes, may also be important risk factors for development of environmentally-associated lupus-like disorders. Awareness of the many environmental agents implicated with lupus and related disorders, and dissection of their pathogenetic mechanisms through appropriate case-controlled investigations, may identify additional toxic agents and may lead to a better understanding of the idiopathic lupus syndromes.