How to proxy http_x_ssl_client_s_dn header?

I want to proxy the request header 'HTTP_X_SSL_CLIENT_S_DN' through nginx.

Here is our server network structure.

[front server:443] <---> [nginx proxy:8004] <---> [application server:8008]
(client cert auth)

When I tried it two servers([front server] and [application server]), it worked properly. The header 'HTTP_X_SSL_CLIENT_S_DN' was passed to application server.

Then adding the server [nginx proxy], the header 'HTTP_X_SSL_CLIENT_S_DN' was not passed to application server.

My nginx config is below.

server {
    listen   8004;
    index index.html;

    location / {
        proxy_pass_header Server;
        proxy_pass_header X-Scheme;
        proxy_set_header Host $http_host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_pass http://app-server/;

# TODO: to proxy 'HTTP_X_SSL_CLIENT_S_DN'
# failed settings
#        proxy_pass_request_headers on; # not worked (default: on)
#        proxy_pass_header X-SSL-Client-S-DN; # none
#        proxy_pass_header X_SSL_CLIENT_S_DN; # none
#        proxy_pass_header HTTP_X_SSL_CLIENT_S_DN; # none
#        proxy_pass_header HTTP-X-SSL-CLIENT-S-DN; # none
#        proxy_set_header X-SSL-Client-S-DN $ssl_client_s_dn; # none
#        proxy_set_header X_SSL_Client_S_DN $x_ssl_client_s_dn; # none
#        proxy_set_header X-SSL-Client-S-DN $http_ssl_client_s_dn; # none
#        proxy_set_header X-SSL-Client-S-DN $http_x_ssl_client_s_dn; # none
    }
}

upstream app-server {
    server 127.0.0.1:8008;
}

Any help appliciated.