Search our Test Catalog. Specify what to search (name, test code, titles, everywhere) and how to search (begins with, contains)

Unit Code 82964:
X-Linked Hyper IgM Syndrome, Blood

Print Friendly View
Print Page Email Page

Useful For

Screening for XL-HIGM or CD40L deficiency

 

Ascertaining carrier status in mothers of patients diagnosed with  

XL-HIGM

Clinical Information

CD154 (CD40 ligand: CD40L) is required for the interaction of T cells and

B cells as part of the normal adaptive immune response. Activation of

T cells leads to the expression of the CD40L molecule on the cell surface.

CD40L binds the CD40 receptor that is always present on B cells,

monocytes, and macrophages (regardless of environmental conditions).

This interaction of CD40L with CD40 is important in B cell proliferation,

differentiation, and class-switch recombination (isotype class-switching).

 

Patients with X-linked hyper-IgM (XL-HIGM) syndrome have defective

CD40L expression on their activated helper CD4 T cells.(1,2) This leads

to defective B-cell responses and the absence of immunoglobulin

class-switching. These features are typified in these patients by a

profound reduction or absence of isotype class-switched memory B cells

(CD19 CD27 IgM-IgD-) with low or absent secreted IgG and IgA, and

normal or elevated serum IgM levels.(1,2) Due to the impairment of

T-cell function and macrophage activation, XL-HIGM patients are

particularly prone to opportunistic infections with Pneumocystis jiroveci,

Cryptosporidium, and Toxoplasma gondii.(1)

 

To date, more than 100 unique mutations of CD40LG, the gene that

encodes CD40L, have been described, affecting the intracellular,

transmembrane, and, more commonly, extracellular domain containing

the CD40-binding region.

 

A defect in surface expression of CD40L on activated CD4 T cells can be

demonstrated using an anti-CD40L antibody and flow cytometry.(3,4)

Since certain CD40LG mutations can maintain surface protein expression,

albeit with loss of function, it is important to also evaluate CD40L-binding

capacity to eliminate the possibility of false-negative results. A soluble

recombinant, chimeric receptor protein, CD40-uIg, is incorporated into the

assay, which assesses CD40L function by determining receptor-binding

activity. Approximately 20% of XL-HIGM patients have activated CD4 T

cells with normal surface expression of CD40L, but aberrant function.(4)

Reference Values

Present

Interpretation

This is a qualitative assay; CD40L-protein expression and function is

reported as present or absent. Absence of CD40L-protein expression

and function is consistent with XL-HIGM. In females, the presence of 2

populations-normal and abnormal-is consistent with carrier status.

 

Most patients with XL-HIGM have absent or significantly reduced CD40L

expression on their activated CD4 T cells. Patients with normal CD40L

expression, but abnormal function, show an absence of binding with

soluble chimeric CD40-uIg antibody, substantiating a diagnosis of

XL-HIGM. Females who are carriers for this disease will show a typical

bimodal pattern of CD40L expression, with 50% of the T cells lacking any

CD40L expression. In the case of aberrant protein function, a similar

profile will be obtained with the CD40-uIg antibody.

 

CD69 is a marker for T-cell activation and serves as a positive control; in

the absence of induced CD69 expression on T cells, the presence of

XL-HIGM cannot be assessed.

Cautions

The test must be performed on fresh, heparinized whole blood cells for

appropriate CD40L expression on activated CD4 T cells; specimen

handling instructions must be followed. T-cell activation is variable on

specimens tested between 48 and 72 hours after blood collection. These

specimens will be analyzed and results will be reported after the laboratory

director's review. Specimens received more than 72 hours after collection

will be rejected and the assay will not be performed.

 

Patients with normal CD40L expression and normal receptor binding with

the CD40-uIg antibody, yet presenting with the clinical phenotype of HIGM

syndrome, should be evaluated for autosomal recessive forms of this

syndrome, including mutations in CD40, AICDA (AID), and UNG.(1,2) A

combination of clinical features and laboratory analyses should permit

identification of an underlying HIGM defect, if present.

 

The other form of XL-HIGM involving mutations in the NEMO (NF-kappa B

essential modulator) gene (official symbol IKBKG) can be easily

discriminated from the CD40LG deficiency due to the unusual and

characteristic clinical findings including abnormal development of

ectoderm-derived skin structures and immunodeficiency with increased

susceptibility to mycobacterial infections.(1,2)

 

Previous studies have reported mutations involving splice sites that result

in the generation of small amounts of wild-type CD40L, associated with a

milder clinical phenotype.(4) In these cases, the CD40-uIg fusion protein

may show some binding, albeit at lower intensity and, therefore, the final

molecular diagnosis depends on sequencing of the CD40LG gene.

 

This is not a confirmatory test for CD40L deficiency, and genetic testing

must be performed to determine the specific mutation involved. Information

about genetic testing for CD40L deficiency is available by contacting Mayo

Laboratory Inquiry at 800-533-1710.

Clinical Reference

1.   Etzioni A, Ochs HD: The hyper IgM syndrome-an evolving story.

      Pediatr Res 2004:56(4):519-525

 

2.   Durandy A, Peron S, Fischer A: Hyper-IgM syndromes. Curr Opin

      Rheumatol 2006;18(4):369-376

 

3.   Lee WI, Torgerson TR, Schumacher MJ, et al: Molecular analysis of a

      large cohort of patients with the hyper immunoglobulin M (IgM)

      syndrome. Blood 2005;105(5):1881-1890

 

4.   Seyama K, Nonoyama S, Gangsaas I, et al: Mutations of the CD40

      ligand gene and its effect on CD40 ligand expression in patients with

      X-linked hyper IgM syndrome. Blood 1998;92:2421-2434

Key